Use more portable #!/usr/bin/env bash shebang

 - Update `caf` and `cafrun` to use the more portable shebang
   `#!/usr/bin/env bash` rather than the old `#!/usr/bin/bash`
 - The user/system bash will get used

Author: zbeekman

src/extensions/caf-head

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Coarray Fortran (CAF) Compiler Wrapper
 #

src/extensions/cafrun-head

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Coarray Fortran (CAF) Executable Launcher
 #