Fix build.psm1 to not add tool path to $PATH twice

[daxian-dbw]

PR Summary

It incorrectly uses $env:XXX -notcontains <path> to test if a string is in the environment variable.
-notcontains works on collections, not a single string. It should be changed to -not $env:XXX.Contains(<path>) instead.

PR Checklist

• PR has a meaningful title
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• Summarized changes
• Change is not breaking
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• Testing - New and feature
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  • Make sure you've added a new test if existing tests do not effectively test the code changed
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