ci: cache Helm binary in CI to eliminate per-job download overhead

- Added HELM_VERSION: "v3.16.4" to the workflow global environment to enable DRY key construction for caching.
- Introduced actions/cache@v3 (id: helm-cache) before the "Install Helm CLI" step in the unit_tests, integration_tests, and helm jobs to cache the Helm binary and reduce per-job download overhead (~15-25 seconds per job).
- Made the "Install Helm CLI" step conditional on cache misses by using steps.helm-cache.outputs.cache-hit != 'true' in all three jobs.
- Removed the hardcoded HELM_VERSION from the install script; it now relies on the HELM_VERSION environment variable.
- Preserved checksum verification on cache miss to maintain integrity of the cached binary.

ISSUES CLOSED: #10033

.forgejo/workflows/ci.yml

@@ -10,6 +10,7 @@ env:
     UV_VERSION: "0.8.0"
     PYTHON_VERSION: "3.13"
     NOX_DEFAULT_VENV_BACKEND: "uv"
+    HELM_VERSION: "v3.16.4"
 
 jobs:
     lint:
@@ -185,9 +186,16 @@ jobs:
               run: |
                   apt-get update && apt-get install -y -qq nodejs git curl tar && rm -rf /var/lib/apt/lists/*
 
+            - name: Cache Helm binary
+              uses: actions/cache@v3
+              id: helm-cache
+              with:
+                  path: /usr/local/bin/helm
+                  key: helm-${{ env.HELM_VERSION }}-linux-amd64
+
             - name: Install Helm CLI
+              if: steps.helm-cache.outputs.cache-hit != 'true'
               run: |
-                  HELM_VERSION="v3.16.4"
                   ARCH="amd64"
                   HELM_TARBALL="helm-${HELM_VERSION}-linux-${ARCH}.tar.gz"
                   curl -fsSL "https://get.helm.sh/${HELM_TARBALL}" -o "/tmp/${HELM_TARBALL}"
@@ -196,7 +204,8 @@ jobs:
                   tar -xzf "/tmp/${HELM_TARBALL}" -C /tmp
                   mv /tmp/linux-${ARCH}/helm /usr/local/bin/helm
                   chmod +x /usr/local/bin/helm
-                  helm version --short
+              env:
+                  HELM_VERSION: ${{ env.HELM_VERSION }}
 
             - uses: actions/checkout@v4
 
@@ -236,9 +245,16 @@ jobs:
               run: |
                   apt-get update && apt-get install -y -qq nodejs git curl tar && rm -rf /var/lib/apt/lists/*
 
+            - name: Cache Helm binary
+              uses: actions/cache@v3
+              id: helm-cache
+              with:
+                  path: /usr/local/bin/helm
+                  key: helm-${{ env.HELM_VERSION }}-linux-amd64
+
             - name: Install Helm CLI
+              if: steps.helm-cache.outputs.cache-hit != 'true'
               run: |
-                  HELM_VERSION="v3.16.4"
                   ARCH="amd64"
                   HELM_TARBALL="helm-${HELM_VERSION}-linux-${ARCH}.tar.gz"
                   curl -fsSL "https://get.helm.sh/${HELM_TARBALL}" -o "/tmp/${HELM_TARBALL}"
@@ -247,7 +263,8 @@ jobs:
                   tar -xzf "/tmp/${HELM_TARBALL}" -C /tmp
                   mv /tmp/linux-${ARCH}/helm /usr/local/bin/helm
                   chmod +x /usr/local/bin/helm
-                  helm version --short
+              env:
+                  HELM_VERSION: ${{ env.HELM_VERSION }}
 
             - uses: actions/checkout@v4
 
@@ -473,9 +490,16 @@ jobs:
 
             - uses: actions/checkout@v4
 
+            - name: Cache Helm binary
+              uses: actions/cache@v3
+              id: helm-cache
+              with:
+                  path: /usr/local/bin/helm
+                  key: helm-${{ env.HELM_VERSION }}-linux-amd64
+
             - name: Install Helm CLI
+              if: steps.helm-cache.outputs.cache-hit != 'true'
               run: |
-                  HELM_VERSION="v3.16.4"
                   ARCH="amd64"
                   HELM_TARBALL="helm-${HELM_VERSION}-linux-${ARCH}.tar.gz"
                   curl -fsSL "https://get.helm.sh/${HELM_TARBALL}" -o "/tmp/${HELM_TARBALL}"
@@ -484,7 +508,8 @@ jobs:
                   tar -xzf "/tmp/${HELM_TARBALL}" -C /tmp
                   mv /tmp/linux-${ARCH}/helm /usr/local/bin/helm
                   chmod +x /usr/local/bin/helm
-                  helm version --short
+              env:
+                  HELM_VERSION: ${{ env.HELM_VERSION }}
 
             - name: Install kubeconform
               run: |