`uv-cli` recompiles unnecessarily in worktree

[eth3lbert]

Currently, when working in a worktree, uv-cli still recompiles even when there are no changes. Running cargo check --verbose multiple times shows something similar to the following::

...
       Fresh backtrace v0.3.73
       Fresh uv-dispatch v0.0.1 (/Users/eth/workspace/astral-sh/uv-dev/crates/uv-dispatch)
       Fresh tikv-jemalloc-sys v0.6.0+5.3.0-1-ge13ca993e8ccb9ba9847cc330696e02839f328f7
       Dirty uv-cli v0.0.1 (/Users/eth/workspace/astral-sh/uv-dev/crates/uv-cli): the file `.git/HEAD` is missing
   Compiling uv-cli v0.0.1 (/Users/eth/workspace/astral-sh/uv-dev/crates/uv-cli)
       Fresh nix v0.28.0
       Fresh clap_complete_command v0.6.1
       Fresh console v0.15.8
     Running `/Users/eth/workspace/astral-sh/uv-dev/target/debug/build/uv-cli-aeca9008583611ce/build-script-build`
       Fresh is_ci v1.2.0
       Fresh tikv-jemallocator v0.6.0
       Fresh ctrlc v3.4.4
...

This occurs because a worktree's .git is a file, not a directory, which means there is no .git/HEAD file for the worktree.