Fix coverage job on self-hosted runners by installing nextest. (#39895)

`./mach test-unit` now requires nextest.
`./mach bootstrap` is skipped on self-hosted runners, which don't have
nextest installed yet, so we need to install it manually. The action is
also much faster than installing from source, so this also benefits the
github-hosted coverage job.

Testing: Not needed, this just installs another tool in CI.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

.github/workflows/linux.yml

@@ -401,7 +401,7 @@ jobs:
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@v2
         with:
-          tool: cargo-llvm-cov
+          tool: cargo-llvm-cov,cargo-nextest
       - name: Bootstrap dependencies
         if: ${{ runner.environment != 'self-hosted' }}
         shell: bash