Auto merge of #16370 - aneeshusa:enable-using-local-states-for-mach-bootstrap, r=jdm

Add env var for local saltfs root during bootstrap

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This enables using the environment variable `SERVO_SALTFS_ROOT` to use a local copy of the saltfs tree for `./mach bootstrap`. The immediate use case is to add a test for `./mach bootstrap` to saltfs itself, to ensure Salt changes or refactors don't break `./mach bootstrap`.

I tested this locally in an Ubuntu Trusty VM.

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- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because the test will be added to saltfs

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