Build MV3 variant for Chrome's beta channel

In order to begin testing the MV3 variant in the beta channel
distributed in the Chrome Web Store.

Change-Id: I0dad1f3b5e3b3b0b17fe2e88ce7b7212f2cfcc83
diff --git a/Makefile b/Makefile
index 3fdb95e..4b501c0 100644
--- a/Makefile
+++ b/Makefile
@@ -41,9 +41,9 @@
 	rm -rf dist/chromium
 
 release_chromium_beta: deps
-	$(WEBPACK) --mode production --env browser_target=chromium
-	$(RELEASE_SCRIPT) -c beta -b chromium
-	rm -rf dist/chromium
+	$(WEBPACK) --mode production --env browser_target=chromium_mv3
+	$(RELEASE_SCRIPT) -c beta -b chromium_mv3
+	rm -rf dist/chromium_mv3
 
 release_gecko_stable: deps
 	$(WEBPACK) --mode production --env browser_target=gecko