Fix brioche self-update setting wrong permissions

[kylewlacy]

brioche self-update updates Brioche itself to the latest version. It does so by downloading a manifest, downloading a linked file, validating the hash, and overwriting its own binary with the linked file.

Currently, if an update is published, the step where Brioche overwrites its own binary doesn't set permissions properly. Once the update is complete, the Brioche executable will no longer have the executable permission, effectively bricking the installation without manual user intervention.

(Luckily, I anticipated an "unknown unknown" breakage in the update process, and made it so the manifest can print a message without performing an update, letting users know that an update is available and needs to be manually installed)

This bug is present in 0.1.0 and 0.1.1. This also means the next update will require manual installation as well.

The fix should be pretty straightforward: ensure that the executable bit is set properly when overwriting the Brioche executable.