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.

test/functional/core/env_spec.lua

@@ -0,0 +1,27 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local eq = t.eq
+local clear = n.clear
+local exec_capture = n.exec_capture
+local command = n.command
+
+describe('vim.uv', function()
+  before_each(function()
+    clear()
+  end)
+
+  -- Subsequential env var assignment consistency
+  -- see: issue 32550
+  it('vim.uv.os_setenv(), vim.uv.os_unsetenv() consistency', function()
+    eq('', exec_capture('echo $FOO'))
+    command('lua vim.uv.os_setenv("FOO", "bar")')
+    eq('bar', exec_capture('echo $FOO'))
+    command('lua vim.uv.os_setenv("FOO", "fizz")')
+    eq('fizz', exec_capture('echo $FOO'))
+    command('lua vim.uv.os_unsetenv("FOO")')
+    eq('', exec_capture('echo $FOO'))
+    command('lua vim.uv.os_setenv("FOO", "buzz")')
+    eq('buzz', exec_capture('echo $FOO'))
+  end)
+end)