fix(env.c): drop envmap, free os_getenv() result #32683

Problem:
vim.uv.os_setenv gets "stuck" per-key. #32550
Caused by the internal `envmap` cache. #7920

    :echo $FOO  <-- prints nothing
    :lua vim.uv.os_setenv("FOO", "bar")
    :echo $FOO  <-- prints bar (as expected)
    :lua vim.uv.os_setenv("FOO", "fizz")
    :echo $FOO  <-- prints bar, still (not expected. Should be "fizz")
    :lua vim.uv.os_unsetenv("FOO")
    :echo $FOO  <-- prints bar, still (not expected. Should be nothing)
    :lua vim.uv.os_setenv("FOO", "buzz")
    :echo $FOO  <-- prints bar, still (not expected. Should be "buzz")

Solution:
- Remove the `envmap` cache.
  - Callers to `os_getenv` must free the result.
- Update all call-sites.
- Introduce `os_getenv_noalloc`.
- Extend `os_env_exists()` the `nonempty` parameter.

src/nvim/diff.c

@@ -1143,7 +1143,7 @@ static int diff_file(diffio_T *dio)
   char *const cmd = xmalloc(len);
 
   // We don't want $DIFF_OPTIONS to get in the way.
-  if (os_getenv("DIFF_OPTIONS")) {
+  if (os_env_exists("DIFF_OPTIONS", true)) {
     os_unsetenv("DIFF_OPTIONS");
   }