Auto merge of #12020 - crepererum:fix/shebang, r=aneeshusa

Add shebang to Linux runservo.sh

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Shebang is missing and therefore `./runservo.sh` does not work under some environments.

I've chosen to use `/usr/bin/env sh` instead of `/bin/sh` or `/usr/bin/sh` to auto-select the right `sh` executable depending on the environment.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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- [X] These changes do not require tests because it does not affect servo itself but rather the resulting package under some environments.

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