Use printchplenv from CHPL_HOME, if set, for nightly emails.
Use printchplenv from CHPL_HOME, if set, for nightly emails.

Previously, the nightly emails would run printchplenv relative to the
nightly/nightly_email.pl script. This works fine when the compiler is located
in the same location as the scripts. If the compiler is somewhere else (for
example in a binary installation like homebrew or cray module), however, it
would still use the version next to nightly/nightly_email.pl script instead of
the printchplenv used by the compiler.

Update nightlysubs.pm to check for CHPL_HOME in the environment, and if it
exists use it to find printchplenv. When CHPL_HOME is not set in the
environment, it will go back to the old behavior of using the relative path.

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