Refactor S3 asset URL handling (#1085)

Previously the buildpack's S3 bucket was defined in two places - once
in `VENDOR_URL` and again during the pip installation step. This
duplication was necessary since `VENDOR_URL` also contained the stack's
name, whereas the pip use-case used a non-stack-specific S3 key prefix.

In order to:
* reduce this duplication
* simplify this buildpack's S3 bucket migration (where we'll soon be
  needing the vary the bucket name and wouldn't want to have to
  duplicate that logic in multiple places)
* allow overriding of the URL for the pip use-case

...the `VENDOR_URL` variable has been replaced with `S3_BASE_URL` which
no longer contains the stack name.

The user-configurable override has similarly been renamed from
`BUILDPACK_VENDOR_URL` to `BUILDPACK_S3_BASE_URL`. Note: As before,
this override cannot be set via standard app variables (see #989).

The unused `USE_STAGING_BINARIES` environment variable has been
removed, since it's a leftover from the project to stand up a staging S3 bucket.
It's redundant given the `BUILDPACK_S3_BASE_URL` variable.

Closes @W-8142401@.

test/run-features

@@ -113,10 +113,10 @@ testHooks() {
     PROFILE_PATH
     PWD
     PYTHONUNBUFFERED
+    S3_BASE_URL
     SHLVL
     SOME_APP_CONFIG_VAR
     STACK
-    VENDOR_URL
   )
   if [[ "${STACK}" == "cedar-14" || "${STACK}" == "heroku-16" ]]; then
     # Remove "OLDPWD" from expected_env_vars since for bash <4.4 it's not exported to subshells: