chpl_launchcmd: Use cleaner form for testing env var.
chpl_launchcmd: Use cleaner form for testing env var.

Suggested by @ronawho

chizarlicious/chapel,CoryMcCartan/chapel,chizarlicious/chapel,hildeth/chapel,chizarlicious/chapel,hildeth/chapel,CoryMcCartan/chapel,chizarlicious/chapel,CoryMcCartan/chapel,hildeth/chapel,CoryMcCartan/chapel,chizarlicious/chapel,hildeth/chapel,hildeth/chapel,hildeth/chapel,chizarlicious/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel,hildeth/chapel,chizarlicious/chapel,CoryMcCartan/chapel