smoke-test, os-check: pin CCACHE_DIR to the cached path

The two jobs that manage their ccache cache manually rely on ccache's
XDG default (~/.cache/ccache) matching the actions/cache path. That
holds today, but nothing enforces it: a later change that sets
CCACHE_DIR (e.g. adopting the ccache-setup composite, which uses
~/.ccache) would silently decouple the build's cache from the
saved/restored directory. Pin CCACHE_DIR explicitly to the cached
path so the pairing is visible and cannot drift.

.github/workflows/os-check.yml

@@ -75,6 +75,12 @@ jobs:
       - name: Allow unprivileged user namespaces (for bwrap)
         run: sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0 || true
 
+      # ccache's default cache dir (XDG ~/.cache/ccache) is what the
+      # actions/cache step below saves; pin it explicitly so the two
+      # cannot drift apart (e.g. if a later change sets CCACHE_DIR).
+      - name: Pin ccache directory
+        run: echo "CCACHE_DIR=$HOME/.cache/ccache" >> "$GITHUB_ENV"
+
       - name: Restore ccache
         uses: actions/cache@v4
         with:

.github/workflows/smoke-test.yml

@@ -98,6 +98,13 @@ jobs:
         if: steps.merge_check.outputs.skip != 'true'
         run: sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0 || true
 
+      # ccache's default cache dir (XDG ~/.cache/ccache) is what the
+      # actions/cache step below saves; pin it explicitly so the two
+      # cannot drift apart (e.g. if a later change sets CCACHE_DIR).
+      - name: Pin ccache directory
+        if: steps.merge_check.outputs.skip != 'true'
+        run: echo "CCACHE_DIR=$HOME/.cache/ccache" >> "$GITHUB_ENV"
+
       - name: Restore ccache
         if: steps.merge_check.outputs.skip != 'true'
         uses: actions/cache@v4