Reduce memory pressure while running tests in CI (#4161)
Reduce memory pressure while running tests in CI (#4161)

Motivation:

We are getting `OutOfMemoryError` during the build on a CI machine
spontaneously. According to the heap dump, ProGuard keeps the data
required for shading JARs indefinitely, incurring memory pressure on the
Gradle JVM.

Modifications:

- Run `shadedJar` and `shadedTestJar` tasks in a separate step so that
  other tasks are unaffected by them.

Result:

Hopefully no `OOME` during CI
kojilin/armeria,kojilin/armeria,minwoox/armeria,kojilin/armeria,trustin/armeria,kojilin/armeria,minwoox/armeria,trustin/armeria,line/armeria,trustin/armeria,line/armeria,minwoox/armeria,trustin/armeria,minwoox/armeria,line/armeria,line/armeria,minwoox/armeria,kojilin/armeria,kojilin/armeria,line/armeria,trustin/armeria,minwoox/armeria,trustin/armeria,line/armeria