ci(desktop): bump rust-cache prefix so release-ci deps actually cache

[brettchien]

The problem

#51 added a release-ci profile to cut the sidecar build from ~23m, but it never actually got faster — its own bundle-macos run took 19m51s on the sidecar step (Cache hit ... full match: true yet a full cold compile), and the post-merge run on main is doing the same right now.

Root cause

Swatinem/rust-cache keys the cache on Cargo.lock (+ rustc + cargo config), not on Cargo.toml's [profile.*] sections. Verified empirically: a pre-#51 run (no release-ci in Cargo.toml) and #51's run share the identical key v0-rust-bundle-macos-Darwin-arm64-f9b08cb2-282619ef. So every run:

1. Gets a full match: true against the old v0 cache — which only holds deps compiled under target/.../release/, never release-ci/.
2. cargo build --profile release-ci writes to a fresh release-ci/ dir → recompiles the whole aws-sdk / aws-lc-sys / ring tree cold (~20m) every run.
3. Because the key matched, the post step logs Cache up-to-date and saves nothing. GitHub cache keys are immutable, so the release-ci deps are never persisted — the fast path can never arrive.

The fix

prefix-key: "v1-rust"
key: ${{ hashFiles('Cargo.toml') }}

• prefix-key bump — one-time reset off the poisoned v0 cache. First run (this PR + first main push) misses → one cold build → actually saves the release-ci deps. After that, restores hit and cargo's own fingerprinting recompiles only oab-mcp itself → minutes, the goal of ci(desktop): build the oab-mcp sidecar with a fast profile (~23min → minutes) #51.
• key: hashFiles('Cargo.toml') — folds the profile-defining manifest into the cache key so this is self-healing: any future [profile.*] edit auto-mints a fresh key instead of silently reusing a stale slot (the exact trap ci(desktop): build the oab-mcp sidecar with a fast profile (~23min → minutes) #51 fell into). No need to hand-bump prefix-key again.

Chosen over a hand-rolled actions/cache on the sidecar binary because that needs a manual invalidation key — and oab-mcp depends on the workspace crate studio-cp, so a naive hashFiles('crates/oab-mcp/**') key would ship a stale sidecar when studio-cp changes. Letting cargo decide keeps correctness.

Verification

• This PR's bundle-macos run: sidecar step is the first-ever v1 build → expect ~20m (cold, expected) and a cache save at the end (post step should NOT say "Cache up-to-date").
• Next push to main after this merges should restore the v1 cache and drop the sidecar step to minutes.

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