CI: avoid unnecessary work when testing flambda2

[xclerc]

When runtest-upstream is run for the flambda2 compiler,
and parallel is not present the following command:

$(MAKE) --no-print-directory exec-one DIR=$$LINE

results in:

$(MAKE) --no-print-directory exec-one DIR= some/path

because the file read to set LINE has leading spaces on
all lines.

As a consequence, the following lines:

	  for dir in $(DIR)/*; do
	    if [ -d $$dir ]; then
	      $(MAKE) exec-one DIR=$$dir;
	    fi;
	  done;

are evaluated with DIR set to the empty string, hence
iterating over the whole file system from the root.

As far as I can tell, it does not happen when parallel
is present because parallel itself appears to be
trimming the strings.