Minor updates
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix#28905 (GitHub issue number if applicable)
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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they are very small changes
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Fix simpleservo binary check on macos
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When compiling libsimpleservo on MacOS 11.5.2 I would get an error at the end of the process saying
```
Error running mach:
['build', '-d', '--libsimpleservo']
The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.
If filing a bug, please include the full output of mach, including this error
message.
The details of the failure are as follows:
servo.command_base.BuildNotFound: No Servo binary found. Perhaps you forgot to run `./mach build`?
File "/Users/yuriwitte/_HtmlTests/servo/python/servo/build_commands.py", line 734, in build
self.get_binary_path(release, dev, target=target, simpleservo=libsimpleservo)
File "/Users/yuriwitte/_HtmlTests/servo/python/servo/command_base.py", line 394, in get_binary_path
raise BuildNotFound('No Servo binary found.'
```
The binary is there, it's just that `get_binary_path` on `command_base.py` checks for a libsimpleservo **.so** instead of a **.dylib**. This pr fixes this check.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix: didn't find a specific issue for this, but these two are also related to file extensions on MacOS:
https://github.com/servo/servo/issues/27654https://github.com/servo/servo/issues/27318
<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because: they _are_ the tests I guess?
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add NixOS support to mach and automatically re-launch mach in nix-shell
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This is a followup for #28454 which adds nixos support directly to mach, saving the user from typing `nix-shell` everytime
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)
<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they just extend mach
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