community/mupdf: remove bashism in prepare

This bashism is not supported by busybox ash. Busybox interprets this as
a single file name and due to the -f flag being passed this does not
result in an error.

This regression was introduced in 010d7fa4.
7 jobs for !7002 with mupdf-fix-prepare in 3 minutes and 9 seconds
latest detached
Status Job ID Name Coverage
  Lint
failed #97125
docker-alpine x86_64 allowed to fail
lint

00:00:22

 
  Build
passed #97130
docker-alpine ci-build aarch64
build-aarch64

00:01:15

passed #97131
docker-alpine ci-build armv7
build-armv7

00:02:47

passed #97129
docker-alpine ci-build ppc64le
build-ppc64le

00:01:45

passed #97128
docker-alpine ci-build s390x
build-s390x

00:00:57

passed #97127
docker-alpine ci-build x86
build-x86

00:01:02

passed #97126
docker-alpine ci-build x86_64
build-x86_64

00:00:51

 
Name Stage Failure
failed
lint Lint
No job log