UBports Installer 0.4.20-beta

A quick bugfix relase of the UBports Installer.

Changelog:

• Fix issue where some devices could not be oem-unlocked
• Fix issue where some oem-unlocked devices would not be detected as such
• Migrate to core20 base snap
• Remove unsupported device placeholders from API
• Fix issue where installing empty channels resulted in cryptic errors
• implement a fastboot:format step to be used in the installer config files
• The installer is currently experiencing problems with Proxies. Add a note to the connection-lost screen about this.

A snap package is available by running snap install ubports-installer on any Linux distribution.