Run Cygwin CI workflow commands in login shells

[EliahKagan]

This passes --login to the bash shell used to run commands in the Cygwin environment on CI. This eliminates the need to work around a partly broken environment, and the extra code what was used to do that is accordingly removed. There are two benefits of this change:

• The PATH is correct: Cygwin's /usr/local/bin and /usr/bin are present at the beginning of PATH. Otherwise, it is easy to get /usr/bin at the front, but rather involved to get /usr/local/bin to precede it. Because Python on Cygwin puts scripts/executables such as the upgraded pip and the pytest command in /usr/local/bin, it is valuable to have that directory in the PATH and best to have it before /usr/bin. (I have set CYGWIN_NOWINPATH to omit other directories, since finding any of the commands to be run in the Cygwin environment outside that environment is unintended.)

• Every step automatically has correct temporary directories: When Cygwin commands were not being run in login shells, they didn't automatically get correct values for TMP and TEMP for their environment. To work around this, those environment variables were set globally, for every step. But that caused them to refer to nonexistent locations for steps such as actions/checkout. Most likely this would not cause any errors, but it did cause copious warnings about a nonexistent temporary directory, which risked obscuring other potentially important output. Now that Cygwin commands run in login shells, both the few non-Cygwin steps, and the steps run in the Cygwin environment, all get correct temporary directories (with TMP and TEMP set in the prewritten startup script the login shell uses).

A theoretical disadvantage of this is that login shells take slightly longer to start up, but that delay is insignificant in this application. A more significant disadvantage is that setting the -x shell option the way it was done before would produce a lot of noise at the beginning of the output for every command-running step. To work around that, -x is omitted from the value of shell and set -x is added at the end of the startup script for login shells, so it runs before each step's "payload" command, but without applying to the commands run in the startup script itself.

[Byron]

That's so awesome! Great discovery!

I can sense that native Windows support is just around the corner :).