$MYVIMRC is an environment variable that Vim adds to the current environment.
Environment variables are usually inherited by child processes from their parent process. For example, you can see
$MYVIMRC in a shell that is a child process of Vim:
:!printenv | grep MYVIMRC
(I use this form rather than just
:!echo "$MYVIMRC" to "prove" that the value is not inserted before the command is sent to the shell.)
I don't use lua in Vim but I know you can examine environment variables with
os.getenv() so, for instance, this will print the expected value:
In other words, you don't need any special Vim API to get the value.