fix: downgrading image-webpack-loader to fix linux arch bug

There’s a documented issue in “imagemin-gifsicle” where a recent update (5.0.0) removed the binaries for linux distributions:

imagemin/gifsicle-bin#113

This causes it to explode in CI via our direct dependency on image-webpack-loader.  The 7.0.0 version of image-webpack-loader relies on version 5 of gifsicle.  If we downgrade back to 6.0.0, we get gifsicle 4.0.1 instead, which still has the binaries.

package.json

@@ -56,7 +56,7 @@
     "html-webpack-new-relic-plugin": "2.0.0",
     "html-webpack-plugin": "4.5.0",
     "identity-obj-proxy": "3.0.0",
-    "image-webpack-loader": "7.0.1",
+    "image-webpack-loader": "6.0.0",
     "jest": "26.4.2",
     "mini-css-extract-plugin": "0.12.0",
     "new-relic-source-map-webpack-plugin": "1.2.0",