Fix macOS tmpdir cleanup race caused by system daemons creating ~/Library

[sean-]

On macOS, system daemons (cfprefsd, lsd) automatically create $HOME/Library/ for any process whose HOME they observe. Since the build env sets HOME=tmpDir, these daemons can recreate Library/ inside a target's temporary directory during or just after cleanup, causing os.RemoveAll to fail with ENOTEMPTY.

Handle this with a platform-specific cleanTmpDir that detects the Library/ directory on ENOTEMPTY and retries removal with bounded backoff. Non-Darwin platforms get a zero-overhead passthrough.

Without this, building plz on macOS would result in output like:

[snip]
Messages:
15:00:16.029 WARNING: Failed to remove temporary directory for //src/fs:_fs_test#modinfo: unlinkat plz-out/tmp/src/fs/_fs_test#modinfo._build: directory not empty
15:00:33.455 WARNING: Failed to create cache directory /Users/sean/Library/Caches/please/third_party/go/_std#srcs/Q5JCAr0UcVqFPYe2uwoh6JPawUY=: rename /Users/sean/Library/Caches/please/third_party/go/_std#srcs
/Q5JCAr0UcVqFPYe2uwoh6JPawUY== /Users/sean/Library/Caches/please/third_party/go/_std#srcs/Q5JCAr0UcVqFPYe2uwoh6JPawUY=: file exists
15:01:15.399 WARNING: Failed to setup cache directory: unlinkat /Users/sean/Library/Caches/please/third_party/go/std/Pnp1rXnQL2Z78LQJ3bPhTXFeIu0==/std/pkg/darwin_amd64/crypto: directory not empty

Which, while harmless, is not helpful for developers starting to use plz.