Disabled SHELL=dash on Windows due to invalid path backslash handling

Calling an executable via full path with Windows backslashes fails in
dash.

src/test/run-make-fulldeps/tools.mk

@@ -30,9 +30,11 @@ DIFF := diff -u --strip-trailing-cr
 # whenever possible. Most developer platforms include `/bin/dash`, but to ensure
 # tests still work when `/bin/dash`, if not available, this `SHELL` override is
 # conditional:
+ifndef IS_WINDOWS # dash interprets backslashes in executable paths incorrectly
 ifneq (,$(wildcard /bin/dash))
 SHELL := /bin/dash
 endif
+endif
 
 # This is the name of the binary we will generate and run; use this
 # e.g. for `$(CC) -o $(RUN_BINFILE)`.