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.
10 jobs for 284-unify-keyfile-to-configuration-conversions-in-system-tests in 8 minutes and 12 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Precheck
passed #18273
amd64 docker linux
precheck:debian:sid:amd64

00:00:12

 
  Build
passed #18274
amd64 docker linux
build:clang:debian:sid:amd64

00:02:56

passed #18276
i386 docker linux
build:clang:debian:sid:i386

00:03:14

passed #18275
amd64 docker linux
build:debian:sid:amd64

00:01:46

passed #18277
i386 docker linux
build:debian:sid:i386

00:01:49

 
  Test
passed #18281
amd64 docker linux
systemtest:debian:sid:amd64

00:04:43

passed #18282
i386 docker linux
systemtest:debian:sid:i386

00:04:39

passed #18279
amd64 docker linux
unittest:clang:debian:sid:amd64

00:00:46

passed #18278
amd64 docker linux
unittest:debian:sid:amd64

00:00:47

passed #18280
i386 docker linux
unittest:debian:sid:i386

00:00:40