Consider switching CI to GitHub Actions

[lnicola]

From #23:

"C:\windows\system32\cmd.exe" /D /E:ON /V:OFF /S /C "CALL "D:\a\_temp\516e97b5-bcac-4955-874a-8beaf4abf6c9.cmd""
info: profile set to 'default'
info: default host triple is i686-pc-windows-msvc
error: toolchain 'stable' is not installed
error: caused by: not a directory: 'C:\Users\VssAdministrator\.rustup\toolchains\stable-i686-pc-windows-msvc'
##[error]Cmd.exe exited with code '1'.
Finishing: Install rust

##[warning]An image label with the label macOS-10.13 does not exist.
,##[error]The remote provider was unable to process the request.

[matklad]

yeah, this is kind of unfortunate... I was thinking even about moving this back to rust-analyzer repo. I think github actions allow one to gate CI on changes to specific files, so we might be ably to avoid iffy CI for all PR which don't touch the vfs