tools: add release host var to promotion script

Adds a `RELEASE_HOST` environment variable that enable releasers to
provide a custom proxy host to connect to when performing the promotion
steps of a given release.

Signed-off-by: Ruy Adorno <ruyadorno@google.com>

doc/contributing/releases.md

@@ -827,6 +827,14 @@ or at runtime with:
 $ ./tools/release.sh -i ~/.ssh/node_id_rsa
 ```
 
+You can also specify a different ssh server address to connect to by defining
+a `RELEASE_HOST` environment variable:
+
+```console
+# Substitute proxy.xyz with whatever address you intend to use
+$ RELEASE_HOST=proxy.xyz ./tools/release.sh
+```
+
 `tools/release.sh` will perform the following actions when run:
 
 <details>

tools/release.sh

@@ -8,7 +8,11 @@
 
 set -e
 
-webhost=direct.nodejs.org
+if [ "$RELEASE_HOST" = "" ]; then
+  RELEASE_HOST=direct.nodejs.org
+fi
+
+webhost=$RELEASE_HOST
 webuser=dist
 promotablecmd=dist-promotable
 promotecmd=dist-promote