Fix detecting CLion execution by using os.getenv("_") (which should c…

[nspo]

…ontain "clion")

See also https://github.com/SeanCurtis-TRI/drake_gdb/issues/2 and https://unix.stackexchange.com/questions/292991/usage-of-as-an-environment-variable-passed-to-a-command?rq=1

[peci1]

This works for me in 2019 RC on Linux, thanks ;)