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/path.c

@@ -2392,7 +2392,7 @@ bool path_is_absolute(const char *fname)
 void path_guess_exepath(const char *argv0, char *buf, size_t bufsize)
   FUNC_ATTR_NONNULL_ALL
 {
-  const char *path = os_getenv("PATH");
+  char *path = os_getenv("PATH");
 
   if (path == NULL || path_is_absolute(argv0)) {
     xstrlcpy(buf, argv0, bufsize);
@@ -2427,4 +2427,5 @@ void path_guess_exepath(const char *argv0, char *buf, size_t bufsize)
     // Not found in $PATH, fall back to argv0.
     xstrlcpy(buf, argv0, bufsize);
   }
+  xfree(path);
 }