fix(ci): use --workspace so release builds darktide-ffi artifacts

[ModifAmorphic]

Summary

Hotfix for the v0.2.0 release failure. The packaging step failed with:

tar: target/release/libdarktide_ffi.so: Cannot stat: No such file or directory
tar: target/release/libdarktide_ffi.a: Cannot stat: No such file or directory

Root cause

This repo has a root [package] (darktide-extractor-cli) plus a [workspace] declaration. In that layout, plain cargo build defaults to building only the root package and its dependencies (darktide-bundle). darktide-ffi is a sibling workspace member that nothing depends on, so cargo silently skips it. The dtex binary builds fine, but no FFI artifacts are produced.

Reproduced locally before fixing:

$ cargo build --release
   Compiling darktide-bundle v0.2.0
   Compiling darktide-extractor-cli v0.2.0
    Finished `release` profile [optimized] target(s) in 0.83s
$ ls target/release/libdarktide_ffi.*
ls: cannot access 'target/release/libdarktide_ffi.*': No such file or directory

Fix

Add --workspace to force cargo to build every workspace member, including darktide-ffi's cdylib + staticlib + rlib targets:

run: cargo build --release --workspace

Verified locally that the expected artifacts (libdarktide_ffi.so, libdarktide_ffi.a, dtex) are produced.

Note on v0.2.0

v0.2.0 was already tagged and a (prerelease) GitHub Release was created with zero assets. This fix unblocks future releases but does not retroactively backfill v0.2.0. Recovery options for v0.2.0 specifically will be discussed separately — likely either delete the v0.2.0 release+tag and let release-please re-cut on the next feat/fix commit, or leave v0.2.0 as a prerelease with no assets and let v0.2.1 be the first properly-shipped release.

Verification

• cargo build --release --workspace produces target/release/libdarktide_ffi.so, target/release/libdarktide_ffi.a, and target/release/dtex.
• YAML validates cleanly.
• No code changes; CI-only fix.