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/main.c

@@ -938,12 +938,11 @@ static void remote_request(mparm_T *params, int remote_args, char *server_addr,
     if (!chan) {
       fprintf(stderr, "Remote ui failed to start: %s\n", connect_error);
       os_exit(1);
-    } else if (strequal(server_addr, os_getenv("NVIM"))) {
+    } else if (strequal(server_addr, os_getenv_noalloc("NVIM"))) {
       fprintf(stderr, "%s", "Cannot attach UI of :terminal child to its parent. ");
       fprintf(stderr, "%s\n", "(Unset $NVIM to skip this check)");
       os_exit(1);
     }
-
     ui_client_channel_id = chan;
     return;
   }
@@ -2118,7 +2117,7 @@ static void source_startup_scripts(const mparm_T *const parmp)
 static int execute_env(char *env)
   FUNC_ATTR_NONNULL_ALL
 {
-  const char *initstr = os_getenv(env);
+  char *initstr = os_getenv(env);
   if (initstr == NULL) {
     return FAIL;
   }
@@ -2132,6 +2131,8 @@ static int execute_env(char *env)
 
   estack_pop();
   current_sctx = save_current_sctx;
+
+  xfree(initstr);
   return OK;
 }