Teach the Ebitengine build about our own module namespace

inkwell is now a published module (engines/inkwell v0.1.0) rather than a
sibling checkout, so a project that depends on it needs GOPRIVATE — our
forge is not proxy.golang.org and not in the public checksum database.
Set in both the builder image and the Makefile template, so it holds
whether or not the image has been rebuilt.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

example-makefile.make

@@ -4,6 +4,10 @@
 
 PROJECT = ebitenginedemo
 
+# Our own modules live on the forge, not on proxy.golang.org: fetch them
+# directly and skip the public checksum database.
+export GOPRIVATE = git.teletypegames.org
+
 BIN_DIR     = bin
 DIST_DIR    = dist
 WASM_NAME   = game.wasm