- `env.ZIG_GLOBAL_CACHE_DIR` does not resolve variables, so
all actions had their cache dir in the current directory, named
'$TMPDIR/zig-cache'.
- zig cache does not conflict, so it's totally fine to reuse it in both
build and tests (thus removing `--cache-dir`).
While we're at this, fix a `substituteInPlace` deprecation warning.
Motiejus Jakštys
5cdff541c3