Use the correct name of the VTEST-GCOV indicating variable.

Not sure where things went wrong on this...

bin/varnishtest/tests/v00004.vtc

@@ -3,7 +3,7 @@ varnishtest "test if our default paramers make sense ..."
 feature 64bit
 
 # Skip this test in GCOV mode, 68xx bytes extra per level makes it fail
-feature cmd {test -z "$GCOV_PROG"}
+feature cmd {test -z "$GCOVPROG"}
 
 # ... for our definition of a standard use case:
 # - 2019 header madness