After replacing the value of revision in media/libdav1d/moz.yaml with a random value to force update:

> ./mach vendor media/libdav1d/moz.yaml -r 6c6d25d355b78556d231b1a5633ded2ddb9e3774 --ignore-modified
MozillaBuild Install Directory: C:\mozilla-build\
Usage: grep [OPTION]... PATTERN [FILE]...
Try `grep --help' for more information.
unable to start ssh-agent service, error :1058
Error connecting to agent: No such file or directory
 0:02.02 Starting new HTTPS connection (1): code.videolan.org
 0:02.23 Latest commit is 6c6d25d355b78556d231b1a5633ded2ddb9e3774 from 2021-03-15T14:38:41.000+02:00
 0:02.24 Fetching code archive from https://code.videolan.org/videolan/dav1d/-/archive/6c6d25d355b78556d231b1a5633ded2ddb9e3774.tar.gz
 0:02.24 Starting new HTTPS connection (1): code.videolan.org
 0:03.67 rm -rf third_party/dav1d
 0:03.86 Unpacking upstream files from third_party/dav1d.
Error running mach:

    ['vendor', 'media/libdav1d/moz.yaml', '-r', '6c6d25d355b78556d231b1a5633ded2ddb9e3774', '--ignore-modified']

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 vendor| 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:

PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'C:\\Users\\Kagami\\AppData\\Local\\Temp\\tmpq7antrvl'

  File "C:\Users\Kagami\Documents\GitHub\gecko-dev\python/mozbuild/mozbuild/vendor/mach_commands.py", line 91, in vendor

    vendor_command.vendor(library, manifest, revision, check_for_update)
  File "C:\Users\Kagami\Documents\GitHub\gecko-dev\python/mozbuild\mozbuild\vendor\vendor_manifest.py", line 57, in vendor
    self.fetch_and_unpack(commit)
  File "C:\Users\Kagami\Documents\GitHub\gecko-dev\python/mozbuild\mozbuild\vendor\vendor_manifest.py", line 154, in fetch_and_unpack
    mozfile.remove(tardir)
  File "C:\Users\Kagami\Documents\GitHub\gecko-dev\testing/mozbase/mozfile\mozfile\mozfile.py", line 528, in __exit__
    os.unlink(self.__dict__["_path"])

Sentry event ID: 40fa6a196e044aefa804edc47ce3c34c
Sentry is attempting to send 0 pending error messages
Waiting up to 2 seconds
Press Ctrl-Break to quit

https://hg.mozilla.org/mozilla-central/rev/3e349af4587a