ci: remove RUST_TEST_THREADS=1 from musl job

The PTY_LOCK mutex in pty_terminal now serializes the entire PTY
session (spawn, I/O, and cleanup) on musl, so single-threaded test
execution is no longer needed. Non-PTY tests run in parallel.

https://claude.ai/code/session_011H8UR3gS6hoyQAf2x7Dfw8

Author: claude

.github/workflows/ci.yml

@@ -149,9 +149,6 @@ jobs:
       # -crt-static: vite-task is shipped as a NAPI module in vite+, and musl Node
       # with native modules links to musl libc dynamically, so we must do the same.
       RUSTFLAGS: --cfg tokio_unstable -D warnings -C target-feature=-crt-static
-      # On musl, concurrent PTY operations can trigger SIGSEGV in musl internals.
-      # Run test threads sequentially to avoid the race.
-      RUST_TEST_THREADS: 1
     steps:
       - name: Install Alpine dependencies
         shell: sh {0}