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 2cad3825)
8 jobs for 284-unify-keyfile-to-configuration-conversions-in-system-tests-v9_11 in 10 minutes and 7 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Precheck
passed #18307
amd64 docker linux
precheck:debian:sid:amd64

00:00:11

 
  Build
passed #18308
amd64 docker linux
build:debian:sid:amd64

00:01:29

passed #18309
i386 docker linux
build:debian:sid:i386

00:01:38

 
  Test
passed #18312
amd64 docker linux
systemtest:debian:sid:amd64

00:04:09

passed #18314
linux docker i386
systemtest:debian:sid:i386

00:04:07

passed #18310
amd64 docker linux
unittest:debian:sid:amd64

00:00:55

passed #18311
i386 docker linux
unittest:debian:sid:i386

00:00:49

failed #18313
i386 docker linux
systemtest:debian:sid:i386

00:06:08