add hard-coded public gpg key to build

Author: bjornregnell

build.sbt

@@ -72,3 +72,7 @@ ThisBuild / publishMavenStyle := true
 //https://oss.sonatype.org/#stagingRepositories
 //https://oss.sonatype.org/#nexus-search;quick~se.lth.cs
 //https://repo1.maven.org/maven2/se/lth/cs/introprog_2.12/
+
+usePgpKeyHex("E7232FE8B8357EEC786315FE821738D92B63C95F")
+
+//https://github.com/sbt/sbt-pgp#configuration-signing-key