Cherry-pick GASNet bug4723 fix : ssh-spawner failures with tcsh (#25313)

Cherry-pick [GASNet
fix](https://bitbucket.org/berkeleylab/gasnet/commits/76c8a6a3904d10fe3e9a6d2f3442cf810d00c5a5)
to [bug
4723](https://gasnet-bugs.lbl.gov/bugzilla/show_bug.cgi?id=4723), which
resolves ssh-spawner failures when using tcsh.

Problem was reproduced on JLSE cascade, and fix confirmed using the
patched Chapel build tree.

[Reviewed by @jhh67]

third-party/gasnet/README

@@ -34,4 +34,4 @@ Chapel modifications to GASNet
 The modifications that we have made to the official GASNet release are
 as follows:
 
-* None
+* Cherry-picked 76c8a6a39 - Address bug 4723 in which ssh-spawner chokes tcsh

third-party/gasnet/gasnet-src/other/spawner/gasnetrun.pl

@@ -192,6 +192,11 @@ ($)
 # Implement -E for ssh, if required, as a wrapper (processed below)
     if (($spawner eq 'SSH') && defined $envlist) {
         foreach (split(',', $envlist)) {
+            # Screen both variable names and values for "bad" ones.
+            # See Bug 4723 for the motivation.
+            next if ($ENV{$_} =~ m/\n/); # skip value with newline(s), which we cannot portably quote
+            next unless ($_ =~ m/^[A-Za-z_][A-Za-z0-9_]*$/); # skip invalid variable name
+
             unshift @ARGV, "$_=$ENV{$_}";
         }
         unshift @ARGV, $ENV{'GASNET_ENVCMD'};