fix: improve version detection fallback in build.rs

[ralphmodales]

Fix version detection in build.rs for cargo-binstall installations

This PR implements improvements suggested in issue #2025.

The main change is fixing the version detection logic in build.rs to properly handle cases where Git operations fail or return empty results. Previously, when using cargo-binstall, the gix --version command would output nothing because the build script was not properly falling back to CARGO_PKG_VERSION.

Changes

• Improved Git command validation: Added explicit check for output.status.success() to ensure Git command actually succeeded
• Fixed empty string handling: Modified parse_describe() to return None for empty strings instead of Some(""), allowing proper fallback to CARGO_PKG_VERSION
• Enhanced error handling: Build script now properly detects when Git describe fails and falls back to package version

Issue

When installing via cargo-binstall, the pre-built binaries were compiled in environments where:

• Git commands may have failed silently
• Repository tags were not available
• The fallback mechanism was not triggering due to empty strings being treated as valid results

The original parse_describe function would return Some("") for empty Git output, preventing the unwrap_or_else fallback from executing.

Testing

• Verified that gix --version now properly displays version information when installed via cargo-binstall
• Confirmed that source builds continue to work as expected
• Version detection now works correctly in both scenarios

Fixes #2025

[Byron]

Great, thank so much!