ci(cross): do not restore target/ in the windows->linux job

The job builds twice — once for the host, then once for x86_64-linux-musl — and
a cache-restored BMI tree makes the second build read std BMIs that no longer
match what the dependency BMIs were compiled against:

    mcpplibs.cmdline: error: import 'std' has CRC mismatch

GCC bakes a CRC of each imported module's BMI into the importer, so the two have
to come from the same build round. A cache that restores one without the other
is not a partial speedup, it is an unbuildable tree.

Systematic, not flaky: it reproduced on rerun, and only in this job. The other
two cross jobs cache ~/.mcpp and ~/.xlings but deliberately never target/ —
this now follows the same convention.

It stayed hidden until a PR touched neither mcpp.toml nor .xlings.json, since
those two files key the sandbox cache; every earlier run had been a cold miss.
That also means it would have reddened every subsequent PR, so it is fixed
ahead of the B3 work rather than alongside it.

Author: Sunrisepeak

.github/workflows/cross-build-test.yml

@@ -307,6 +307,25 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/bootstrap-mcpp
+        with:
+          # Do NOT restore target/ in a cross job. This job builds twice — once
+          # for the host, then once for x86_64-linux-musl — and a restored BMI
+          # tree makes the second build read `std` BMIs that no longer match
+          # what the dependency BMIs were compiled against:
+          #
+          #   mcpplibs.cmdline: error: import 'std' has CRC mismatch
+          #
+          # GCC bakes a CRC of each imported module's BMI into the importer, so
+          # the two have to come from the same build round; a cache that
+          # restores one without the other is not a partial speedup, it is an
+          # unbuildable tree. It reproduced on rerun, and only in this job —
+          # the other two cross jobs below cache ~/.mcpp and ~/.xlings but
+          # deliberately never target/, which is the convention this now follows.
+          #
+          # It stayed hidden until a PR touched neither mcpp.toml nor
+          # .xlings.json: those two files key the sandbox cache, so every
+          # earlier run had been a cold miss.
+          cache-target: 'false'
 
       - name: Build mcpp from source (self-host)
         shell: bash