tests: don't hard-code /bin/bash stable

tests: don't hard-code /bin/bash

From Thomas Klausner in pkgsrc.

Differential Revision: https://phab.mercurial-scm.org/D10507

--- a/tests/test-transaction-rollback-on-sigpipe.t	Wed Apr 21 17:59:14 2021 +0200
+++ b/tests/test-transaction-rollback-on-sigpipe.t	Thu Apr 22 02:57:30 2021 +0200
@@ -22,14 +22,14 @@
 
   $ killable_pipe=`pwd`/killable_pipe.sh
   $ script $killable_pipe <<EOF
-  > #!/bin/bash
+  > #!/usr/bin/env bash
   > echo \$\$ >> $pidfile
   > exec cat
   > EOF
 
   $ remotecmd=`pwd`/remotecmd.sh
   $ script $remotecmd <<EOF
-  > #!/bin/bash
+  > #!/usr/bin/env bash
   > hg "\$@" 1> >($killable_pipe) 2> >($killable_pipe >&2)
   > EOF
 
@@ -38,7 +38,7 @@
 
   $ hook_script=`pwd`/pretxnchangegroup.sh
   $ script $hook_script <<EOF
-  > #!/bin/bash
+  > #!/usr/bin/env bash
   > for pid in \$(cat $pidfile) ; do
   >   kill \$pid
   >   while kill -0 \$pid 2>/dev/null ; do