Add helper variables in mkeys system test

The keyfile and key ID for the original managed key do not change
throughout the mkeys system test.  Keep them in helper variables to
prevent calling "cat" multiple times and improve code readability.

(cherry picked from commit 68f056b2)
7 jobs for 284-unify-keyfile-to-configuration-conversions-in-system-tests-v9_12 in 6 minutes and 50 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Precheck
passed #18283
amd64 docker linux
precheck:debian:sid:amd64

00:00:11

 
  Build
passed #18284
amd64 docker linux
build:debian:sid:amd64

00:01:47

passed #18285
i386 docker linux
build:debian:sid:i386

00:01:55

 
  Test
passed #18288
amd64 docker linux
systemtest:debian:sid:amd64

00:04:42

passed #18289
i386 docker linux
systemtest:debian:sid:i386

00:04:42

passed #18286
amd64 docker linux
unittest:debian:sid:amd64

00:00:48

passed #18287
i386 docker linux
unittest:debian:sid:i386

00:00:44