fix(gatsby-plugin-cxs): Publish plugin files properly (#11902)

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## Description
`gatsby-ssr.js` is not included in the NPM package. Due to two rules below in `.npmignore`
```
/*.js
!/index.js
```
After PR, the file will be published to the npm.

## Related Issues

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  Link to the issue that is fixed by this PR (if there is one)
  e.g. Fixes #1234, Addresses #1234, Related to #1234, etc.
-->
N/A

packages/gatsby-plugin-cxs/.npmignore

@@ -32,5 +32,3 @@ flow-typed
 coverage
 decls
 examples
-/*.js
-!/index.js