add PLTUSERHOME

The new `PLTUSERHOME` environment variable redirects all of the
user-specific paths reported by `find-system-path`.

Also, improve the tests for `raco exe` (particularly the bug
fixed in 6cb6f3fbf1) using `PLTUSERHOME`.

original commit: e4ce0d0331

pkgs/compiler-pkgs/compiler-test/tests/compiler/embed/test.rkt

@@ -53,7 +53,12 @@
   (let ([plthome (getenv "PLTHOME")]
 	[collects (getenv "PLTCOLLECTS")]
         [out (open-output-string)])
+    (define temp-home-dir (make-temporary-file "racket-tmp-home~a" 'directory))
     ;; Try to hide usual collections:
+    (parameterize ([current-environment-variables
+                    (environment-variables-copy
+                     (current-environment-variables))])
+      (putenv "PLTUSERHOME" (path->string temp-home-dir))
       (when plthome
         (putenv "PLTHOME" (path->string (build-path (find-system-path 'temp-dir) "NOPE"))))
       (when collects
@@ -70,11 +75,8 @@
           (test #t
                 path
                 (parameterize ([current-output-port out])
-                (system* path)))))
-    (when plthome
-      (putenv "PLTHOME" plthome))
-    (when collects
-      (putenv "PLTCOLLECTS" collects))
+                  (system* path))))))
+    (delete-directory/files temp-home-dir)
     (let ([stdout-file (build-path (find-system-path 'temp-dir) "stdout")])
       (if (file-exists? stdout-file)
           (test expect with-input-from-file stdout-file