Don't overwrite BUILDKITE_METAHOOK_HOOKS_PATH

If we have multiple `metahook` instances, let's not generate multiple
temporary directories; instead just create one (and only dump the
environment variables once).

This fixed an issue with multiple `metahook` instances in a pipeline of
mine.

hooks/environment

@@ -1,9 +1,11 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-BUILDKITE_METAHOOK_HOOKS_PATH="$(mktemp -d)"
-export BUILDKITE_METAHOOK_HOOKS_PATH
-env | sort | grep "BUILDKITE_PLUGIN_METAHOOK" | uniq >"${BUILDKITE_METAHOOK_HOOKS_PATH}/vars"
+# If users add multiple `metahook` instances, we'll just store our `vars` in the same directory.
+if [[ ! -v "BUILDKITE_METAHOOK_HOOKS_PATH" ]]; then
+  export BUILDKITE_METAHOOK_HOOKS_PATH="$(mktemp -d)"
+  env | sort | grep "BUILDKITE_PLUGIN_METAHOOK" | uniq >"${BUILDKITE_METAHOOK_HOOKS_PATH}/vars"
+fi
 
 if grep -E 'BUILDKITE_PLUGIN_METAHOOK_.+(\.BAT|\.CMD)=' "${BUILDKITE_METAHOOK_HOOKS_PATH}/vars"; then
   echo "Sorry, we had to remove Windows Batch file support in 0.4.0."