ci: drop cache: 'npm' from setup-node (silent abort on Windows) (#5474)

`actions/setup-node@v6.4.0` with `cache: 'npm'` silently aborts the
entire job on Windows runners when the npm cache path returned by
`npm config get cache` (`C:\npm\cache`) does not yet exist on a fresh
runner -- the step exits 24s in with no error message and every
following step gets skipped. See npm/cli#7308 for the underlying
EEXIST / missing-dir race in the npm cache directory.

This mirrors the existing precedent in
`studio-windows-ui-smoke.yml`'s `setup-python` block, which already
dropped `cache: 'pip'` for the same reason (post-step fatal error on
a missing pip cache dir). The frontend `npm ci` is fast enough
without the cache that the reliability gain is worth the ~30s.

.github/workflows/security-audit.yml

@@ -137,8 +137,6 @@ jobs:
       - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e  # v6.4.0
         with:
           node-version: '22'
-          cache: 'npm'
-          cache-dependency-path: studio/frontend/package-lock.json
 
       - uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8  # stable @ 2026-03-27
 
@@ -1066,8 +1064,6 @@ jobs:
       - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e  # v6.4.0
         with:
           node-version: '22'
-          cache: 'npm'
-          cache-dependency-path: studio/frontend/package-lock.json
 
       - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405  # v6.2.0
         with: