Use encrypted codecov token.
The Cirrus CI config included the codecov token in cleartext as I was under the
impression that this was necessary for coverage reports on pull requests to work
correctly. But this seems to be also causing pushes in other people's
Elvish repositories to appear in the master branch's commit history
(https://app.codecov.io/gh/elves/elvish/commits). Changing the token to be
encrypted might fix the problem; if this breaks coverage reports on pull
requests the token can be switched back to cleartext.
Qi Xiao