ci: revert gapic-error-conformance plugin execution

The gapic-error-conformance tool expects an executable binary or
script for the -plugin flag and fails with 'no such file or directory'
when passed a string like 'node file.js'.

This commit reverts the workflow to use chmod +x on the generated
plugin script so it can be passed directly to the conformance tool,
since we previously updated the typescript code to ensure the script
itself is generated with execution permissions.

.github/workflows/generator-tests.yaml

@@ -63,7 +63,8 @@ jobs:
         curl -sSL https://github.com/googleapis/gapic-config-validator/releases/download/v0.6.0/gapic-config-validator-0.6.0-linux-amd64.tar.gz > config-validator.tar.gz
         tar xzf config-validator.tar.gz --no-same-owner
         chmod +x gapic-error-conformance
-        ./gapic-error-conformance -plugin="node build/typescript/src/protoc-plugin.js"
+        chmod +x build/typescript/src/protoc-plugin.js
+        ./gapic-error-conformance -plugin="build/typescript/src/protoc-plugin.js"
 
     - name: Prepare baseline artifacts
       working-directory: core/generator/gapic-generator-typescript