ARM CI: use GH runner ubuntu-22.04

Use runner ubuntu-22.04 instead of ubuntu-latest. Because ubuntu-latest
has switched to ubuntu-24.04 recently, whiich breaks the build for arm64.

There seem to be an unresolved bug in QEMU >= 8.1
(U24.04 has v8.2.2) when using ARM64 and bullseye:
<https://gitlab.com/qemu-project/qemu/-/issues/2377>.

(ARM64 buster and bookworm, as well as ARMHF, may be fine as indicated
in the bug description.)

This can be reverted later (either when fixed in QEMU or when switched
the chroot to bookworm). But it shouldn't matter much, anyways, since this
is just the "host" environment.

Also include runner ver in chroot key because it seem to make a difference
from the above..

.github/scripts/environment.sh

@@ -107,6 +107,9 @@ if ! is_arm; then
 fi
 
 printf '%b' "FEATURES=$FEATURES\n" >> "$GITHUB_ENV"
+# populate /etc/environment-defined var to global env
+# shellcheck disable=SC2154 # defined by runner in /etc/environment
+printf '%b' "ImageOS=$ImageOS\n" >> "$GITHUB_ENV"
 
 if [ "$(uname -s)" = Darwin ] && [ "$(uname -m)" != arm64 ]; then
         export UG_ARCH=-msse4.2

.github/workflows/arm-build.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   Build:
     if: github.repository == 'CESNET/UltraGrid' || github.event.schedule == null
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04 # TODO: see the commit message why 22.04 is explicit
     strategy:
       matrix:
         arch: [armhf, arm64]
@@ -55,7 +55,9 @@ jobs:
       uses: actions/cache@main
       with:
         path: '~/chroot.tar'
-        key: cache-bootstrap-${{ matrix.arch }}-${{ hashFiles('.github/scripts/Linux/arm/bootstrap.sh', '.github/scripts/Linux/install_others.sh') }}
+        key: cache-bootstrap-${{ env.ImageOS }}-${{ matrix.arch }}-${{
+          hashFiles('.github/scripts/Linux/arm/bootstrap.sh',
+          '.github/scripts/Linux/install_others.sh') }}
     - name: Debootstrap
       if: steps.cache-bootstrap.outputs.cache-hit != 'true'
       run: |