Only allow absolute paths for EM_CONFIG/EM_CACHE, etc. (#23402)

Allowing relative paths here breaks when emcc is run as a subprocess in
a different directory. This happens in when building system libraries in
batch build mode.

One alternative to doing this would be convert all these paths to
absolute paths when we read them and then re-export them with those
different values for our sub-processes, but that seems more complex and
error prone than just requiring them to be absolute in the first place.

I'm also not sure it makes much sense to use relative paths here since
if the environment contains a relative path then the compiler cannot be
used from different directories. e.g.

```
$ export EM_CACHE=foo
$ emcc hello.c
$ cd foo && emcc foo.c  # confusingly would use a different cache
```

Fixes: #23374

Author: sbc100

test/test_other.py

@@ -15385,6 +15385,11 @@ def test_rust_integration_basics(self):
     }''')
     self.do_runf('main.cpp', 'Hello from rust!', emcc_args=[lib])
 
+  def test_relative_em_cache(self):
+    with env_modify({'EM_CACHE': 'foo'}):
+      err = self.expect_fail([EMCC, '-c', test_file('hello_world.c')])
+      self.assertContained('emcc: error: environment variable EM_CACHE must be an absolute path: foo', err)
+
   @crossplatform
   def test_create_cache_directory(self):
     if config.FROZEN_CACHE:

tools/config.py

@@ -134,8 +134,11 @@ def parse_config_file():
       if env_value in ('', '0'):
         env_value = None
       # Unlike the other keys these two should always be lists.
-      if key in ('JS_ENGINES', 'WASM_ENGINES'):
+      if env_var in ('EM_JS_ENGINES', 'EM_WASM_ENGINES'):
         env_value = env_value.split(',')
+      if env_var in ('EM_CONFIG', 'EM_CACHE', 'EM_PORTS', 'EM_LLVM_ROOT', 'EM_BINARYEN_ROOT'):
+        if not os.path.isabs(env_value):
+          exit_with_error(f'environment variable {env_var} must be an absolute path: {env_value}')
       globals()[key] = env_value
     elif key in config:
       globals()[key] = config[key]