Add self-hosted runners v2 (#4506)

## Issue Addressed

NA

## Proposed Changes

Carries on from #4115, with the following modifications:

1. Self-hosted runners are only enabled if `github.repository == sigp/lighthouse`.
    - This allows forks to still have Github-hosted CI.
    - This gives us a method to switch back to Github-runners if we have extended downtime on self-hosted. 
1. Does not remove any existing dependency builds for Github-hosted runners (e.g., installing the latest Rust).
1. Adds the `WATCH_HOST` environment variable which defines where we expect to find the postgres db in the `watch` tests. This should be set to `host.docker.internal` for the tests to pass on self-hosted runners.

## Additional Info

NA


Co-authored-by: antondlr <anton@delaruelle.net>

watch/tests/tests.rs

@@ -22,6 +22,7 @@ use watch::{
 };
 
 use log::error;
+use std::env;
 use std::net::SocketAddr;
 use std::time::Duration;
 use tokio::{runtime, task::JoinHandle};
@@ -36,6 +37,11 @@ const VALIDATOR_COUNT: usize = 32;
 const SLOTS_PER_EPOCH: u64 = 32;
 const DEFAULT_TIMEOUT: Duration = Duration::from_secs(5);
 
+/// Set this environment variable to use a different hostname for connecting to
+/// the database. Can be set to `host.docker.internal` for docker-in-docker
+/// setups.
+const WATCH_HOST_ENV_VARIABLE: &str = "WATCH_HOST";
+
 fn build_test_config(config: &DatabaseConfig) -> PostgresConfig {
     let mut postgres_config = PostgresConfig::new();
     postgres_config
@@ -71,6 +77,10 @@ pub async fn create_test_database(config: &DatabaseConfig) {
         .expect("Database creation failed");
 }
 
+pub fn get_host_from_env() -> String {
+    env::var(WATCH_HOST_ENV_VARIABLE).unwrap_or_else(|_| "localhost".to_string())
+}
+
 struct TesterBuilder {
     pub harness: BeaconChainHarness<EphemeralHarnessType<E>>,
     pub config: Config,
@@ -107,6 +117,7 @@ impl TesterBuilder {
             database: DatabaseConfig {
                 dbname: random_dbname(),
                 port: database_port,
+                host: get_host_from_env(),
                 ..Default::default()
             },
             server: ServerConfig {