fbt/ufbt: Ensure POSIX paths are passed to GDB on all platforms

[CookiePLMonster]

What's new

GDB heavily dislikes forward slashes from Windows paths and strips them internally instead of normalizing them. Account for this by passing POSIX paths explicitly.

Fixes an issue where GDB was given paths with forward slashes and later stripped them internally, leaving to errors like OSError: Error reading file 'H:devflipper-zero.ufbtcurrentscriptsdebug/STM32WB55_CM4.svd': failed to load external entity "H:devflipper-zero.ufbtcurrentscriptsdebug/STM32WB55_CM4.svd"

Couple of notes:

• This is the first time I see a SCons library in the wild, but far as I can tell, the use of SCons.Node.FS.ProxyEntry is correct here.
• I have not been able to test the entire debugging flow, as only later I realized that debugging through USB without a dev board is not possible. However, I still wanted to fix the issue I spotted. Therefore, my list of fixes may not be exhaustive.

Verification

• Attempt to debug via ufbt debug on Windows.

Checklist (For Reviewer)

• [ ] PR has description of feature/bug or link to Confluence/Jira task
• [ ] Description contains actions to verify feature/bugfix
• [ ] I've built this code, uploaded it to the device and verified feature/bugfix

[CookiePLMonster]

Note - I got my devboard today and ufbt debugging over blackmagic works fine without this PR too - therefore, I'm not sure how severe those issues truly are. I assume Blackmagic's debugging is not actually equivalent to ufbt debug but rather ufbt blackmagic.

[hedger]

@CookiePLMonster could you please try out the current implementation?

[CookiePLMonster]

These changes look good but I'll try it soon and get back to you!