Improve performance of fix-permissions script

[peterisr]

Describe your changes

Using the -exec command {} + (instead of ;) action of find, we can improve the performance of fix-permissions script significantly. The difference between the two is that the former (+) invokes a single process and passes all the files to it as arguments, whereas the latter (;) invokes one process per matched file resulting in thousands of chmod runs per build.

My benchmarks show about a 30 second improvement in steps that involve fix-permissions for the build-all target of make. More importantly for me, user defined Docker images, which use the fix-permissions script, will also show perf improvements.

Issue ticket if applicable

n/a

Checklist (especially for first-time contributors)

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests
• I will try not to use force-push to make the review process easier for reviewers
• I have updated the documentation for significant changes

[mathbunnyru]

Thank you, merging this one 👍