ci: pin nightly to nightly-2026-05-07 for ts-test-suite WASM build

Latest nightly causes rustc to SIGSEGV when compiling tokio for
wasm32-unknown-unknown under -Zbuild-std=std,panic_unwind. PR #171 hits
this in the "Build WASM module" step on both ubuntu-latest and
macos-latest matrix legs of TS Test Suite.

Mirrors the equivalent fix on chore/e2e-ci for hub-client-e2e.yml
(commits d06e120, 245cd25, ff51584):

  - Pin via the workflow's RUSTUP_TOOLCHAIN env var so rust-toolchain.toml
    stays as floating "nightly" for local developers.
  - Switch the dtolnay action to @master with a toolchain input (the
    action no longer accepts @nightly-<date> refs).
  - Request rust-src and wasm32-unknown-unknown explicitly on the pinned
    toolchain. Under floating @nightly rustup auto-installed missing
    components against the active toolchain; once pinned they must be
    requested for that toolchain or the build:wasm step fails with
    "library/Cargo.lock does not exist, unable to build with the
    standard library".

Author: gordonwoodhull

.github/workflows/ts-test-suite.yml

@@ -27,7 +27,13 @@ jobs:
 
     name: Run test suite
     if: github.repository == 'quarto-dev/q2'
-    
+
+    # Pin nightly via env so we don't have to edit rust-toolchain.toml
+    # (which devs may override locally). Latest nightly currently crashes
+    # rustc with SIGSEGV when compiling tokio under -Zbuild-std for wasm32.
+    env:
+      RUSTUP_TOOLCHAIN: nightly-2026-05-07
+
     steps:
       - name: Checkout Repo
         uses: actions/checkout@v6
@@ -48,7 +54,11 @@ jobs:
 
       # Consistent Rust setup for both platforms
       - name: Set up Rust nightly
-        uses: dtolnay/rust-toolchain@nightly
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: nightly-2026-05-07
+          targets: wasm32-unknown-unknown
+          components: rust-src
 
       - name: Output rust version
         shell: bash