patch 8.1.1694: the RUN_VIM variable is longer than needed

Problem:    The RUN_VIM variable is longer than needed.
Solution:   Shorten RUN_VIM. (Daniel Hahler, closes #4643)
diff --git a/src/testdir/Makefile b/src/testdir/Makefile
index 470f41e..0efebb5 100644
--- a/src/testdir/Makefile
+++ b/src/testdir/Makefile
@@ -81,7 +81,7 @@
 
 RM_ON_RUN = test.out X* viminfo
 RM_ON_START = tiny.vim small.vim mbyte.vim mzscheme.vim test.ok benchmark.out
-RUN_VIM = VIMRUNTIME=$(SCRIPTSOURCE); export VIMRUNTIME; $(VALGRIND) $(VIMPROG) -f $(GUI_FLAG) -u unix.vim $(NO_INITS) -s dotest.in
+RUN_VIM = VIMRUNTIME=$(SCRIPTSOURCE) $(VALGRIND) $(VIMPROG) -f $(GUI_FLAG) -u unix.vim $(NO_INITS) -s dotest.in
 
 clean:
 	-rm -rf *.out *.failed *.res *.rej *.orig XfakeHOME Xdir1 Xfind
@@ -150,7 +150,7 @@
 # New style of tests uses Vim script with assert calls.  These are easier
 # to write and a lot easier to read and debug.
 # Limitation: Only works with the +eval feature.
-RUN_VIMTEST = VIMRUNTIME=$(SCRIPTSOURCE); export VIMRUNTIME; $(VALGRIND) $(VIMPROG) -f $(GUI_FLAG) -u unix.vim
+RUN_VIMTEST = VIMRUNTIME=$(SCRIPTSOURCE) $(VALGRIND) $(VIMPROG) -f $(GUI_FLAG) -u unix.vim
 
 newtests: newtestssilent
 	@/bin/sh -c "if test -f messages && grep -q 'SKIPPED\|FAILED' messages; then cat messages; fi"