fix(ci): pass toolchain: stable, not v1, in e2e and chapel-ci

'v1' is the action's tag, not a rustup toolchain; rustup rejects it
('invalid toolchain name: v1'). Same fix the cargo-audit, coverage and
release workflows already carry. Surfaced now that these workflows start.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019a95kk88TZ1iegsBK3kox3

Author: claude

.github/workflows/chapel-ci.yml

@@ -224,7 +224,7 @@ jobs:
       - uses: actions/checkout@v7.0.1
       - uses: dtolnay/rust-toolchain@v1
         with:
-          toolchain: v1
+          toolchain: stable
       - uses: actions/cache@v6.1.0
         with:
           path: |

.github/workflows/e2e.yml

@@ -45,7 +45,7 @@ jobs:
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@v1
         with:
-          toolchain: v1
+          toolchain: stable
 
       - name: Rust cache
         uses: Swatinem/rust-cache@v2.9.2