Auto merge of #22390 - cdeler:fixup-linux-mint-issue-with-bootstrap, r=jdm

Fixup the issue related with an incorrect distro version string handling

The original fixes for #21732 had a mistake.
The fix don't work on the LinuxMint with version 19:
```
  File "./mach", line 96, in <module>
    main(sys.argv)
  File "./mach", line 24, in main
    sys.exit(mach_bootstrap.bootstrap_command_only(topdir))
  File "/home/user/workspace/skv/servo/python/mach_bootstrap.py", line 239, in bootstrap_command_only
    bootstrap(context, force)
  File "/home/user/workspace/skv/servo/python/servo/bootstrap.py", line 380, in bootstrap
    distro, version = get_linux_distribution()
  File "/home/user/workspace/skv/servo/python/servo/bootstrap.py", line 349, in get_linux_distribution
    major, minor = version.split('.')
ValueError: need more than 1 value to unpack
```
I've fixed that

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #21732

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