Support naming the workspace anything other than @rules_prerender

[dgp1130]

Currently, using @rules_prerender means the workspace must be named rules_prerender. This is primarily because of bazel-contrib/bazel-lib#359, where the only current workaround is to hard code the name of the external workspace the target is built within.

However, once that is fixed we should add a test case which deliberately installs the workspace under a different name just to verify that it works as expected.