[3.9] Correct timing sensitivity in iOS testing Makefile target. (pythonGH-118620)

Author: freakboy3742

Makefile.pre.in

@@ -1249,7 +1249,7 @@ testuniversal:	@DEF_MAKE_RULE@ platform
 # a full Xcode install that has an iPhone SE (3rd edition) simulator available.
 # This must be run *after* a `make install` has completed the build. The
 # `--with-framework-name` argument *cannot* be used when configuring the build.
-XCFOLDER=iOSTestbed.$(MULTIARCH).$(shell date +%s)
+XCFOLDER:=iOSTestbed.$(MULTIARCH).$(shell date +%s)
 XCRESULT=$(XCFOLDER)/$(MULTIARCH).xcresult
 .PHONY: testios
 testios: