bootstrap: Allow renaming of fetched bootstrap artifact and don't overwrite Android NDK
Some generally-useful commits extracted from !915 (merged).
.gitignore: Ignore macOS built packages properly
cerbero: Remove some dead code
bootstrap: Allow renaming of fetched bootstrap artifact
This is needed for Rust toolchain bootstrap.
bootstrap: Don't overwrite when extracting the Android NDK
The Android NDK unpack dir is versioned, so overwriting is a waste of
time. The other toolchain files are not versioned, so they need to be
overwritten when, for example, upgrading.