fix: re-tag 27_self_contained_home as unix-shell, revert fresh-sandbox on Windows

Test 27 uses `env -u MCPP_HOME` (POSIX-only) and checks for
`registry/bin/xlings` without .exe. Tag as unix-shell.

Windows fresh-sandbox not reliable yet — xlings LLVM auto-install
in temp dirs has path issues. Revert to skip until resolved.

Author: Sunrisepeak

tests/e2e/27_self_contained_home.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# requires:
+# requires: unix-shell
 # 27_self_contained_home.sh — verifies mcpp's self-contained home behaviour.
 #
 # Without MCPP_HOME set, mcpp resolves its home from the binary's location:

tests/e2e/run_all.sh

@@ -69,7 +69,8 @@ case "$OS" in
         fi
         # NOTE: Windows runners may have g++.exe (MinGW/Strawberry) in PATH
         # but it's not a proper mcpp-compatible GCC. Don't add gcc capability.
-        CAPS+=(fresh-sandbox)
+        # fresh-sandbox: not yet reliable on Windows — xlings LLVM auto-install
+        # into temp MCPP_HOME dirs has path/copy issues. Enable once resolved.
         ;;
 esac