Fix upgrade when channel is not pulled

Fix escaping error from reformatting

Caused: ./flutterw: line 33: cd: "./: No such file or directory

Don't expect a whitespace after .flutter

Fix submodule matching

Merge pull request passsy#7 from agrosner/patch-1

Fix check for existing flutter submodule

Release v1.0.0

Fail fast on download error

Previously the next command failed because flutterw couldn’t be found or executed because the content was “404: Not Found”

Add missing NOT to prepend v for the tag

Merge pull request passsy#3 from passsy/linux_support

Linux support