fix(release/aarch64): two-stage cross-build — bootstrap builds fresh mcpp, then it cross-builds aarch64

The published bootstrap mcpp (0.0.58) predates aarch64 cross-build support
(#146/#148), so 'mcpp build --target aarch64-linux-musl' resolved the x86_64
host musl-gcc and produced no aarch64 binary. Build this release's x86_64 mcpp
first (it has the feature), then cross-build aarch64 with it. Verified locally:
fresh 0.0.59 resolves aarch64-linux-musl-gcc and emits an ARM aarch64 static ELF.

Author: Sunrisepeak

.github/workflows/release.yml

@@ -303,7 +303,14 @@ jobs:
         run: |
           export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
           mcpp self config --mirror GLOBAL 2>/dev/null || true
-          mcpp build --target aarch64-linux-musl
+          # The published bootstrap mcpp predates aarch64 cross-build support
+          # (the feature landed after the last release), so it resolves the
+          # x86_64 host musl-gcc for an aarch64 target. Two-stage instead: build
+          # THIS release's x86_64 mcpp first, then cross-build aarch64 with it.
+          mcpp build --target x86_64-linux-musl
+          FRESH=$(find target/x86_64-linux-musl -type f -name mcpp | head -1)
+          test -n "$FRESH"
+          "$FRESH" build --target aarch64-linux-musl
           BIN=$(find target/aarch64-linux-musl -type f -name mcpp | head -1)
           test -n "$BIN"
           file "$BIN" | grep -q 'ARM aarch64'