CI: pins actions to commit SHAs

A version tag like @v1 or a branch like @master is a movable pointer:
whoever controls the action repository can re-point it, and the next
run executes whatever it points at. Several of these actions run in
jobs whose outputs ship to users: build-release.yml creates the GitHub
release with the downloadable binaries, and build-windows-hosts.yml
holds the SignPath signing token, so a re-pointed tag there could ship
a tampered or wrongly signed release. This is exactly how the
tj-actions/changed-files compromise propagated (CVE-2025-30066).

Pinning by full commit SHA makes the reviewed code the code that runs,
the same way docker/setup-qemu-action is already pinned in
build-linux-vms.yml. Refs that pointed at master (cross-platform-actions,
setup-alpine, get-latest-release) are pinned to their latest release
tag. Every pin keeps a version comment, and each SHA was resolved from
the upstream repository and cross-checked against its release tag.

.github/workflows/build-linux-vms.yml

@@ -15,10 +15,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checkout repository
-        uses: actions/checkout@v7
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
 
       - name: run VM
-        uses: uraimo/run-on-arch-action@v3
+        uses: uraimo/run-on-arch-action@f9b26e3a1a408d5fd530d20c17b9f3f4428ff8d9 # v3.1.0
         id: runcmd
         with:
           arch: ${{ inputs.arch }}
@@ -39,7 +39,7 @@ jobs:
             ctest --output-on-failure
 
       - name: upload artifacts
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
           name: fastfetch-linux-${{ inputs.arch }}
           path: ./fastfetch-*.*