build(Gradle): Configure the distTar task to use GZIP compression

Although contained JARs already are compressed, this reduces the final
archive size by about 20 MiB compared to the plain TAR. As a side note,
BZIP2 compression yields about the same archive size, but is about 4
times slower (5s vs 20s) to compress.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

buildSrc/src/main/kotlin/ort-application-conventions.gradle.kts

@@ -185,6 +185,10 @@ tasks.named<CreateStartScripts>("startScripts") {
     }
 }
 
+tasks.named<Tar>("distTar") {
+    compression = Compression.GZIP
+}
+
 tasks.named<JavaExec>("run") {
     System.getenv("TERM")?.also {
         val mode = it.substringAfter('-', "16color")