[win] Disable MSYS2 path munging when calling is.exe (#46867)

Tragically, I believe MSYS2 is messing with options such as
`/VERYSILENT` turning them instead into `C:\msys2\VERYSILENT` or
similar.

Makefile

@@ -432,8 +432,9 @@ endif
 	cd $(BUILDROOT) && $(TAR) zcvf $(JULIA_BINARYDIST_FILENAME).tar.gz julia-$(JULIA_COMMIT)
 
 exe:
-	# run Inno Setup to compile installer
-	$(call spawn,$(JULIAHOME)/dist-extras/inno/iscc.exe /DAppVersion=$(JULIA_VERSION) /DSourceDir="$(call cygpath_w,$(BUILDROOT)/julia-$(JULIA_COMMIT))" /DRepoDir="$(call cygpath_w,$(JULIAHOME))" /F"$(JULIA_BINARYDIST_FILENAME)" /O"$(call cygpath_w,$(BUILDROOT))" $(INNO_ARGS) $(call cygpath_w,$(JULIAHOME)/contrib/windows/build-installer.iss))
+	# run Inno Setup to compile installer.
+	# Note that we disable MSYS2 path munging, as it interferes with the `/` options:
+	MSYS2_ARG_CONV_EXCL='*' $(call spawn,$(JULIAHOME)/dist-extras/inno/iscc.exe /DAppVersion=$(JULIA_VERSION) /DSourceDir="$(call cygpath_w,$(BUILDROOT)/julia-$(JULIA_COMMIT))" /DRepoDir="$(call cygpath_w,$(JULIAHOME))" /F"$(JULIA_BINARYDIST_FILENAME)" /O"$(call cygpath_w,$(BUILDROOT))" $(INNO_ARGS) $(call cygpath_w,$(JULIAHOME)/contrib/windows/build-installer.iss))
 	chmod a+x "$(BUILDROOT)/$(JULIA_BINARYDIST_FILENAME).exe"
 
 app:
@@ -558,7 +559,7 @@ win-extras:
 	cd $(JULIAHOME)/dist-extras && \
 	$(JLDOWNLOAD) https://www.jrsoftware.org/download.php/is.exe && \
 	chmod a+x is.exe && \
-	$(call spawn, $(JULIAHOME)/dist-extras/is.exe /DIR="$(call cygpath_w,$(JULIAHOME)/dist-extras/inno)" /PORTABLE=1 /CURRENTUSER /VERYSILENT)
+	MSYS2_ARG_CONV_EXCL='*' $(call spawn, $(JULIAHOME)/dist-extras/is.exe /DIR="$(call cygpath_w,$(JULIAHOME)/dist-extras/inno)" /PORTABLE=1 /CURRENTUSER /VERYSILENT)
 
 # various statistics about the build that may interest the user
 ifeq ($(USE_SYSTEM_LLVM), 1)