build(docs-infra): increase build artifact size limit for preview server

In angular#29926, the size of the build artifacts has increased due to turning
on differential loading (which generates an es2015/es5 pair for each JS
resource).

To avoid the preview server's rejecting the build artifacts (as in
[288181][1]), this commit increases the max allowed artifact size from
20MB to 25MB (current artifact size after angular#29926 is ~22MB).

[1]: https://circleci.com/gh/angular/angular/288181

aio/aio-builds-setup/dockerbuild/Dockerfile

@@ -40,7 +40,7 @@ ARG      AIO_TRUSTED_PR_LABEL="aio: preview"
 ARG TEST_AIO_TRUSTED_PR_LABEL="aio: preview"
 ARG      AIO_PREVIEW_SERVER_HOSTNAME=preview.localhost
 ARG TEST_AIO_PREVIEW_SERVER_HOSTNAME=preview.localhost
-ARG      AIO_ARTIFACT_MAX_SIZE=20971520
+ARG      AIO_ARTIFACT_MAX_SIZE=26214400
 ARG TEST_AIO_ARTIFACT_MAX_SIZE=200
 ARG      AIO_PREVIEW_SERVER_PORT=3000
 ARG TEST_AIO_PREVIEW_SERVER_PORT=3001