Update `rapids-size-checker`

[ajschmidt8]

With the addition of the shared-action-workflow changes described below, we should revise the rapids-size-checker to utilize the RAPIDS_BASE_BRANCH environment variable instead of fetching it via the API in the shell script.

• rapids-size-checker lines -

  gha-tools/tools/rapids-size-checker

  Lines 10 to 17 in 4664eb1

  	curl_headers=('-H' "Authorization: token ${RAPIDS_GH_TOKEN}")
  	resp=$(
  	curl \
  	-H "Accept: application/vnd.github.v3+json" \
  	"${curl_headers[@]}" \
  	"https://api.github.com/repos/${GITHUB_REPOSITORY}/pulls/${pr_num}"
  	)
  	base_branch=$(echo "${resp}" | jq -r '.base.ref')

Relevant shared-action-workflow changes:

• Adds RAPIDS_BASE_BRANCH env var - Add custom-job.yaml, RAPIDS_BASE_BRANCH, Codecov support shared-workflows#20
• Reusable composite action - https://github.com/rapidsai/shared-action-workflows/tree/main/get-pr-info