Update Error Prone -XepExcludedPaths flag to be Windows-compatible

[Stephan202]

See #925 (comment): -XepExcludedPaths:\Q${project.build.directory}${file.separator}\E.* doesn't work because Error Prone matches against a path that's derived from a file's URI, which in practice will always contain forward slashes.

Suggested commit message:

Update Error Prone `-XepExcludedPaths` flag to be Windows-compatible (#927)

Prior to these changes the provided pattern would never match on
Windows, as Error Prone matches against file URIs, which in practice
will always contain forward slashes, even on Windows.

Testing:

1. In Add rewrite-templating annotation processor to also produce OpenRewrite recipes #925 I validated that these changes prevent Error Prone from flagging violations against OpenRewrite-generated recipes when running on Windows.
2. (Tested locally on Linux:) Violations in non-generated source code are still reported, as validated by running the build after executing git show a0b1f7091ee4f902f4abdcc9649fe0e902669948 -- error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/CollectorMutability.java | git apply -R.

[github-actions]

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[mohamedsamehsalah]

🚀

[rickie]

Nice fix! Just rebased, will merge once 🟢 .

[sonarqubecloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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[github-actions]

Looks good. No mutations were possible for these changes.
Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[Stephan202]

I updated the label, as this is a build improvement that doesn't impact users :)

[rickie]

Haha had dinner so didn't do the merging 😬. But yeah makes sense.