Fix shebang in 2018/ferguson/test.sh

Make it a bit more portable (assuming env is under /usr/bin):

    #!/usr/bin/env bash

instead of

    #!/bin/bash

And although Landon suggested that I will never be done with changes in
my entries (which is probably true I confess ... :-) ) I suggest that in
this case this is rather useful (in fact I should do this for
2020/ferguson1 as there are several scripts there .. maybe
2020/ferguson2 but I don't know for sure).

2018/ferguson/test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Make sure the program has been compiled
 make clobber all 1> /dev/null|| exit 1