Set .jl sources as read-only during installation (#55524)

This sets all `.jl` files in `$(prefix)/base` and `$(prefix)/test` to
have `0444` permissions, to better match how `Pkg` installs packages
(and sets them to be read-only).

Fixes JuliaLang/juliaup#865

---------

Co-authored-by: Mosè Giordano <765740+giordano@users.noreply.github.com>
(cherry picked from commit 62e7705)

Makefile

@@ -377,6 +377,11 @@ endif
 	cp -R -L $(JULIAHOME)/base/* $(DESTDIR)$(datarootdir)/julia/base
 	cp -R -L $(JULIAHOME)/test/* $(DESTDIR)$(datarootdir)/julia/test
 	cp -R -L $(build_datarootdir)/julia/* $(DESTDIR)$(datarootdir)/julia
+
+	# Set .jl sources as read-only to match package directories
+	find $(DESTDIR)$(datarootdir)/julia/base -type f -name \*.jl -exec chmod 0444 '{}' \;
+	find $(DESTDIR)$(datarootdir)/julia/test -type f -name \*.jl -exec chmod 0444 '{}' \;
+
 	# Copy documentation
 	cp -R -L $(BUILDROOT)/doc/_build/html $(DESTDIR)$(docdir)/
 	# Remove various files which should not be installed