fix(xpkg/test): Replace invalid char in filename

[MisterMX]

Description of your changes

Replace : in filenames with _ (affects a single file).

Fixes https://github.com/upbound/up/issues/424

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [x] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Unit tests remain unchanged. Only the name of file in the OS filesystem is changed. It is kept with : in the in-memory filesystem that is used during tests.

[Upbound-CLA]

All committers have signed the CLA.