fix ale_python_auto_virtualenv to correctly set virtualenv env vars (#4885)

* fix ale_python_auto_virtualenv to correctly set virtualenv env vars

According to the documentation, `ale_python_auto_virtualenv` should automatically set environment variables for commands, but previously the variables were not set completely or correctly.

Before:
  `PATH` variable was expanded to include `/path/to/venv`
After:
  `PATH` variable is expanded to include `/path/to/venv/bin`
  `VIRTUAL_ENV` variable is set to `path/to/venv`

This mimics exactly what the `activate` scripts do, and allows the configuration knob to work as expected.

For example, after this change, `jedi-language-server` can be installed globally (instead of inside every venv), and it will "just work" (e.g. find references to dependencies in the venv) when editing a file in a project that uses a venv, because the correct variables are set.

* fix test_python_virtualenv.vader test to expect output with both virtualenv vars

* remove unnecessary non-escape in test_python_virtualenv.vader

* fix accidentally removed space in windows test_python_virtualenv.vader

test/test_python_virtualenv.vader

@@ -14,10 +14,10 @@ Execute(ale#python#FindVirtualenv falls back to $VIRTUAL_ENV when no directories
 Execute(ale#python#AutoVirtualenvEnvString should return the correct values):
   if has('win32')
     AssertEqual
-    \ 'set PATH=/opt/example/;%PATH% && ',
+    \ 'set PATH=/opt/example/\Scripts;%PATH% && set VIRTUAL_ENV=/opt/example/ && ',
     \ ale#python#AutoVirtualenvEnvString(bufnr(''))
   else
     AssertEqual
-    \ 'PATH=''/opt/example/''":$PATH" ',
+    \ 'PATH=''/opt/example//bin''":$PATH" VIRTUAL_ENV=''/opt/example/'' ',
     \ ale#python#AutoVirtualenvEnvString(bufnr(''))
   endif