fix: Prefix paths in find on macOS (#1284)

This fixes paths not resolving in macOS, causing
the build target to fail. This also renames the
site target to specify the index.html, which is
the output artifact of building the site.

.github/workflows/coder.yaml

@@ -355,7 +355,7 @@ jobs:
             js-${{ runner.os }}-
 
       - name: Build site
-        run: make site/out
+        run: make site/out/index.html
 
       - name: Build Release
         uses: goreleaser/goreleaser-action@v2.9.1
@@ -488,7 +488,7 @@ jobs:
 
       - name: Build
         run: |
-          make site/out
+          make site/out/index.html
 
       - run: yarn playwright:install
         working-directory: site