env depfile: add missing touch (#31494)

Oversight in #31433, the non-phony `env` target was missing a file being
created for it, which can cause make to infinitely loop when including
multiple generated makefiles.

lib/spack/spack/cmd/env.py

@@ -594,6 +594,7 @@ def get_install_target(name):
 {}: {}
 
 {}: {}
+\t@touch $@
 
 {}:
 \t@mkdir -p {}