Honor workspace npm config for CLI installs (#443)

## Summary
- Run npm commands from the caller workspace so npm remains the source
of truth for `.npmrc` parsing, auth, delegated config files, env
expansion, quoting, and precedence.
- Keep the CLI install isolated with `--prefix`, while explicitly
overriding action-owned npm policy such as `offline=false`,
`bin-links=true`, and no package lock.
- Treat existing `node` and `npm` commands on musl as sufficient only
when they actually run, probing Alpine `/usr/bin` binaries with
`/usr/bin` first on PATH.
- Prefer Alpine `/usr/bin` runtime binaries when an earlier PATH entry
shadows them, and keep apk installation for missing runtime libraries or
missing commands.

## Validation
- `bun run ci`
- Real npm sanity check for workspace `.npmrc` with quoted/env delegated
config, `globalconfig`, relative `cafile`, mTLS path keys,
`offline=true`, `bin-links=false`, and package-lock policy
- Fixture workflow:
https://github.com/jgoux/setup-cli-testing/actions/runs/28663875606

Addresses
https://github.com/supabase/setup-cli/pull/442#discussion_r3519786253
Addresses
https://github.com/supabase/setup-cli/pull/442#discussion_r3519786255

action.yml

@@ -61,12 +61,33 @@ runs:
         # actions/setup-node uses glibc Node builds, so install Alpine's Node/npm instead.
         if command -v apk >/dev/null 2>&1; then
           missing_packages=""
-          for package in libstdc++ libgcc nodejs npm; do
+          prefer_apk_bin=false
+          for package in libstdc++ libgcc; do
             if ! apk info -e "${package}" >/dev/null 2>&1; then
               missing_packages="${missing_packages} ${package}"
             fi
           done
 
+          if ! command -v node >/dev/null 2>&1 || ! node -p 'process.versions.node' >/dev/null 2>&1; then
+            if [ -x /usr/bin/node ] && PATH="/usr/bin:${PATH}" /usr/bin/node -p 'process.versions.node' >/dev/null 2>&1; then
+              prefer_apk_bin=true
+            else
+              missing_packages="${missing_packages} nodejs"
+            fi
+          fi
+
+          if ! command -v npm >/dev/null 2>&1 || ! npm --version >/dev/null 2>&1; then
+            if [ -x /usr/bin/npm ] && PATH="/usr/bin:${PATH}" /usr/bin/npm --version >/dev/null 2>&1; then
+              prefer_apk_bin=true
+            else
+              missing_packages="${missing_packages} npm"
+            fi
+          fi
+
+          if [ "${prefer_apk_bin}" = "true" ]; then
+            echo "/usr/bin" >> "$GITHUB_PATH"
+          fi
+
           if [ -z "${missing_packages}" ]; then
             exit 0
           fi
@@ -77,6 +98,9 @@ runs:
           fi
 
           apk add --no-cache ${missing_packages}
+          if echo " ${missing_packages} " | grep -Eq ' (nodejs|npm) '; then
+            echo "/usr/bin" >> "$GITHUB_PATH"
+          fi
           exit 0
         fi