Expose BPLOG_PREFIX to sub-shells again (#1099)

In #1011 the number of buildpack variables that are exported (and so
exposed to subprocesses) was reduced, since in general we don't want
to leak buildpack internals into end-user steps such as the pre/post
compile hooks.

However since this change, any buildpack metric emitted from within
a `sub_env` wrapper (which is a buildpack-stdlib utility function)
is missing its `buildpack.python` prefix.

This is because buildpack-stdlib:
* lazy-loads `BPLOG_PREFIX` (rather than doing so when initially
  sourced, which is the approach used for `BUILDPACK_LOG_FILE`)
* doesn't check whether `BPLOG_PREFIX` is set before emitting metrics

See:
https://github.com/heroku/buildpack-stdlib/blob/v8/stdlib.sh

As a stop-gap until we either fix this in buildpack-stdlib (W-8095466),
or remove usages of the `sub_env` wrapper (since I think they are
counter-productive in this buildpack), I've added back the export
for `BPLOG_PREFIX`.

Fixes @W-8095436@.

test/run-features

@@ -116,6 +116,7 @@ testHooks() {
   local expected_env_vars=(
     _
     BIN_DIR
+    BPLOG_PREFIX
     BUILD_DIR
     BUILDPACK_LOG_FILE
     CACHE_DIR