Refactor build directory purging logic

[IvoBrandao]

Related Issue(s)	2463
Has Unit Tests (y/n)	n
Documentation Included (y/n)	n

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Change Description

This commit refactors the logic for purging the build directory in the run_fbuild_cli function. It now uses a try-except block to handle PermissionError exceptions when attempting to purge the build directory.

Rationale

Fixes Bug

Testing/Review Recommendations

For the testing I did a manual test, the procedure I followed was the following one:

1. Execute `fprime-util generate
2. Execute fprime-util build
3. Change a file to immutable e.g sudo chattr +i build-fprime-automatic-native/lib/Linux/libFw_Buffer.a
4. Execute fprime-util purge
5. The output shows which file cannot be removed as shown below.

[INFO] Purge build directory at: <path>/fprime/testv1/build-fprime-automatic-native
Purge this directory? (yes/no) [yes]: yes
Error: Permission denied while purging <path>/fprime/testprj/build-fprime-automatic-native: [Errno 1] Operation not permitted: 'libFw_Buffer.a'

Future Work

Maybe unit tests, if necessary...

[thomas-bc]

Looks good to me, thank you for working on this! 🚀