ci: publish a draft release from the artifacts that CI already built

The Linux, macOS and Windows workflows build every file that a release
needs when the release branch is pushed. The new Release workflow finds
those runs for the tagged commit, waits for them, and collects their nine
files into a draft release. It builds nothing, so what ships is what was
tested.

The release notes come from the CHANGELOG.md entry for that version.

Signed-off-by: Ron Evans <ron@hybridgroup.com>

.github/workflows/extract-changelog.sh

@@ -0,0 +1,35 @@
+#!/bin/sh
+
+# Print the CHANGELOG.md entry for one version, to use as release notes.
+# The file uses a setext heading: the bare version, then a line of dashes.
+
+set -e
+
+version="$1"
+if [ -z "$version" ]; then
+	echo "usage: $0 <version>" >&2
+	exit 1
+fi
+
+notes=$(awk -v version="$version" '
+	found {
+		if ($0 ~ /^---+$/ && previous != "") { previous = ""; exit }
+		if (previous != "") print previous
+		previous = $0
+		next
+	}
+	$0 ~ /^---+$/ && previous == version {
+		found = 1
+		previous = ""
+		next
+	}
+	{ previous = $0 }
+	END { if (found && previous != "") print previous }
+' CHANGELOG.md)
+
+if [ -z "$notes" ]; then
+	echo "no CHANGELOG.md entry for version $version" >&2
+	exit 1
+fi
+
+echo "$notes"