fix subprocess calls in borg.create

[ftschindler]

Fixes #2219 for me.

Description

Makes the subprocess call more robust, as advised in https://github.com/borgbase/vorta/issues/2219#issuecomment-2771642466

Related Issue

#2219

Motivation and Context

see #2219

How Has This Been Tested?

not at all, just works locally for me, and apparently for @VandalByte as reported in #2219

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

I just did an edit via the Github UI, no prec-ommit or any tests.

• [x] I have read the CONTRIBUTING guide.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.

[m3nu]

Hm. What's the rationale for checking if the command is a file? Are we treating scripts differently with this? What if I put the full path for some command like /sbin/mount?