Default to #!/bin/bash in wrapper scripts on macOS

This gives the user the opportunity to employ BASH_ENV to circumvent
macOS System Integrity Protection and set variables like
DYLD_FRAMEWORK_PATH.

Fixes: QTBUG-57204
Change-Id: Icd99d903a3be76fabd509e204ea61a254a96609c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

mkspecs/features/qt_functions.prf

@@ -159,8 +159,12 @@ defineTest(qtAddToolEnv) {
         !isEmpty(cmd): cmd = "$$cmd "
         equals(ds, /) {
             batch_name = $${batch_name}.sh
+            equals(QMAKE_HOST.os, Darwin):exists(/bin/bash): \
+                shell = /bin/bash
+            else: \
+                shell = /bin/sh
             batch_cont = \
-                "$$LITERAL_HASH!/bin/sh" \
+                "$$LITERAL_HASH!$$shell" \
                 $$batch_sets \
                 "exec $$cmd\"$@\""
             # It would be nicer to use the '.' command (without 'exec' above),