Description
Previously the closing ` was in the wrong place, meaning that nrn_branch was not set correctly and CoreNEURON PRs were always tested against master of NEURON.

Note that in bash:

$ foo=bar | echo hello
hello
$ echo $foo

$

Also tweak the first awk command so that it would not match CI_BRANCHES:FOONEURON_BRANCH=bar.

This should fix a failure in #772.

Also update the GitHub PR template to include a comment describing usage of the CI_BRANCHES keyword.

Use certain branches for the SimulationStack CI
CI_BRANCHES:NEURON_BRANCH=master