feat: Lookup the target cargo directory

[raymondk]

• Instead of moving the wasm, we simply copy it in order to avoid rebuilding unnecessarily
• Lookup the cargo target path so we can tolerate cargo workspaces more easily

[marc0olo]

@raymondk FYI:

There is another nice side effect of the mv → cp change: this also fixes a gzip compression failure on Linux.

cargo build can produce output files with multiple hard links (between target/.../release/ and target/.../deps/). mv on the same filesystem preserves the inode, so the moved file retains those hard links. The subsequent gzip --no-name step then fails with:

gzip: <file> has 1 other link -- unchanged

This only happens with GNU gzip (Linux) — BSD gzip (macOS) doesn't perform this check.

Switching to cp creates a fresh inode with link count 1, so gzip works without issues on both platforms.