ci: Disable library backtrace capture in macOS test jobs (#64555)

Based on
https://github.com/zed-industries/zed/pull/64464#issuecomment-5763255028

With `RUST_BACKTRACE=1` set, `anyhow` captures a backtrace for every
error it constructs; on macOS this costs ~30ms per error, on Linux ~µs.
`Project::test` alone hits several such errors during setup
(`canonicalize`/`load` of the non-existent global `tasks.json` and
`debug.json`, worktree creation, buffer open), adding ~0.8s of fixed
cost per test on macOS.

`RUST_LIB_BACKTRACE=0` is checked by std before `RUST_BACKTRACE`, so
panics keep their backtraces while library error captures stop.
Set only on the macOS test jobs (`run_tests.yml`, `release.yml`), since
Linux and Windows pay nothing for the captures and keep the error-origin
backtraces in failure output.
Measured with `cargo nextest run -p editor --lib`: macOS 119s → 72s,
Linux 78s → 76s (noise).

Trade-off on macOS: a failing `.unwrap()` on an `anyhow::Error` in CI
output loses the backtrace of where the error was constructed; the panic
backtrace at the unwrap site is unaffected.

Release Notes:

- N/A

.github/workflows/release.yml

@@ -14,6 +14,8 @@ jobs:
   run_tests_mac:
     if: (github.repository_owner == 'zed-industries' || github.repository_owner == 'zed-extensions')
     runs-on: namespace-profile-mac-large
+    env:
+      RUST_LIB_BACKTRACE: '0'
     steps:
     - name: steps::checkout_repo
       uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd

.github/workflows/run_tests.yml

@@ -444,6 +444,8 @@ jobs:
     - check_style
     if: needs.orchestrate.outputs.run_tests == 'true' && github.event_name != 'merge_group'
     runs-on: namespace-profile-mac-large
+    env:
+      RUST_LIB_BACKTRACE: '0'
     steps:
     - name: steps::checkout_repo
       uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd