fix: replace panicking expect() with map_or() for edge-case file paths

[srpatcha]

Fix panic on edge-case file paths in LRU disk cache init

Problem

LruDiskCache::init() in src/lru_disk_cache/mod.rs calls .expect("Bad path?") on file.file_name(), which panics if the path ends in .. or is otherwise unusual. A build cache tool should handle unexpected file system states gracefully rather than crashing.

Root Cause

Path::file_name() returns None when the path terminates in .. or consists solely of a root or prefix. The .expect() call converts this into a panic. While uncommon, symlinks or filesystem corruption could produce such paths in the cache directory.

Fix

Replaced .expect("Bad path?").starts_with(TEMPFILE_PREFIX) with .map_or(false, |name| name.starts_with(TEMPFILE_PREFIX)). Paths without a valid file name component are now treated as non-temporary files (skipping the cleanup branch) rather than crashing.

Testing

• Create a cache directory containing a path component ending in ...
• Previously: sccache panics during cache init. Now: the entry is handled gracefully.
• Normal cache operation should be unaffected.

Impact

Affects sccache users whose cache directories may contain unusual file paths. A panic in cache init prevents the entire build cache from working.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.19%. Comparing base (d11e2e0) to head (9ed2e8a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2679      +/-   ##
==========================================
+ Coverage   74.17%   74.19%   +0.01%     
==========================================
  Files          70       70              
  Lines       39207    39206       -1     
==========================================
+ Hits        29083    29087       +4     
+ Misses      10124    10119       -5     

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