ci: prune gcc@10/11 compilers that don't exist on ubuntu-24.04

--reuse pulls buildcache specs compiled with gcc@10. When a package
has no binary (e.g. py-pyyaml) spack falls back to source build and
invokes /usr/bin/gcc-10, which doesn't exist on ubuntu-24.04. Remove
compilers that aren't present on the runner before concretizing so
spack picks gcc@12+ for any source-built fallbacks.

Author: sbryngelson

.github/workflows/spack.yml

@@ -26,6 +26,17 @@ jobs:
                 packages_ref: f0bb11a611fb7f311e788da778208be234601797
                 buildcache: true
 
+          - name: Prune compilers not present on this runner
+            run: |
+                source spack/share/spack/setup-env.sh
+                spack compiler find
+                # ubuntu-24.04 ships gcc-12..14; gcc-10/11 binaries from the
+                # buildcache cannot be used for source builds (no /usr/bin/gcc-10).
+                for v in 10 11; do
+                    spack compiler rm "gcc@${v}" 2>/dev/null || true
+                done
+                spack compilers
+
           - name: Cache Spack install store
             uses: actions/cache@v4
             with: