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@.

CHANGELOG.md

@@ -6,6 +6,7 @@
 ## v183 (2020-10-12)
 
 - Add support for Heroku-20 (#968).
+- Fix metric names for metrics emitted within `sub_env` (#1099).
 
 ## v182 (2020-10-06)
 

bin/compile

@@ -18,7 +18,7 @@ set -eo pipefail
 # Boostrap the Buildpack Standard Library.
 # Disable unused env var warning since shellcheck doesn't know about the stdlib.
 # shellcheck disable=2034
-BPLOG_PREFIX="buildpack.python"
+export BPLOG_PREFIX="buildpack.python"
 export BUILDPACK_LOG_FILE=${BUILDPACK_LOG_FILE:-/dev/null}
 
 [ "$BUILDPACK_XTRACE" ] && set -o xtrace

test/run-features

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