feat: implement replacing previous comments

Based on https://github.com/peter-evans/find-comment

comment_on_pr.sh

@@ -39,6 +39,8 @@ has_elements() {
 # If BASE_REPORT is provided, a comparison will be made
 markdown_from_report() {
 	cat <<-"EOF"
+		<!-- AUTOGENERATED by nix-flake-outputs-size action -->
+
 		# Flake output sizes
 
 		**Definitions:**
@@ -116,28 +118,12 @@ if [ "${CI:-false}" != 'true' ]; then
 	exit 0
 fi
 
-log 'Determine head_ref'
-# For push & tag events it'll bet GITHUB_REF_NAME, for pull_request events it'll be GITHUB_HEAD_REF
-head_ref=${GITHUB_REF_NAME:-$GITHUB_HEAD_REF}
-
-log "Get PR number for $head_ref"
-prs=$(curl -X 'GET' \
-	"$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/pulls?state=open&sort=recentupdate" \
-	-H "Authorization: token $GITHUB_TOKEN" \
-	-H 'accept: application/json')
-
-pr_number=$(echo "$prs" |
-	jq --arg head_ref "$head_ref" '.[] | select(.head.ref == $head_ref) | .number')
-
 # Protect against running before a PR is made or if it is triggered on the main branch
-if [ -z "$pr_number" ]; then
+if [ -z "$PR_ID" ]; then
 	warn "No PR created for this commit"
 	exit 0
 fi
 
-log "Retrieved index: $pr_number"
-log "Expected PR URL: $GITHUB_SERVER_URL/$GITHUB_REPOSITORY/pulls/$pr_number"
-
 log 'Generating comment body'
 comment=$(markdown_from_report "$@")
 
@@ -150,9 +136,20 @@ group 'Request data'
 log "$data"
 endgroup
 
-curl -o - -X 'POST' \
-	"$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/issues/$pr_number/comments" \
-	-H 'accept: application/json' \
-	-H "Authorization: token $GITHUB_TOKEN" \
-	-H 'Content-Type: application/json' \
-	-d "$data"
+if [ -z "$COMMENT_ID" ]; then
+	log 'Posting new comment'
+	curl -o - -X 'POST' \
+		"$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/issues/$PR_ID/comments" \
+		-H 'accept: application/json' \
+		-H "Authorization: token $GITHUB_TOKEN" \
+		-H 'Content-Type: application/json' \
+		-d "$data"
+else
+	log "Editing comment $COMMENT_ID"
+	curl -o - -X 'PATCH' \
+		"$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/issues/$PR_ID/comments/$COMMENT_ID" \
+		-H 'accept: application/json' \
+		-H "Authorization: token $GITHUB_TOKEN" \
+		-H 'Content-Type: application/json' \
+		-d "$data"
+fi