Resolve both GNU tar paths in release script test

[conornash]

Another small macOS one, I'm afraid. On macOS the temporary directory is reached through a symlink (/var points at /private/var), which tripped up one assertion in the release script tests. The release code itself looked fine to me; it was only the test comparing two spellings of the same path.

The below is the agent explanation of what this PR does:

test-release-scripts compared the tar command pathname, built under the unresolved /var/folders/... fixture root, with (truename gtar), which macOS resolves to /private/var/folders/.... The release code was correct; the assertion compared different spellings of the same file.

Both sides are now resolved with platform-truename. With the original fixture root the old assertion evaluates to NIL and the new one to T.