community/grpc: fix CMAKE_INSTALL_PREFIX

Without this change /home/buildozer/aports/community/grpc/pkg/grpc/usr
is used as a prefix in pkgconfig files, e.g.
/usr/lib/pkgconfig/grpc++.pc which is obviously incorrect and causes
application using this pkgconfig file to not compile.

This also requires setting DESTDIR in package() to make sure that the
files are installed to the pkg/ directory.
7 jobs for !13397 with grpc-fix-prefix in 14 minutes and 21 seconds
latest detached
Status Job ID Name Coverage
  Lint
failed #224141
docker-alpine x86_64 allowed to fail
lint

00:00:23

 
  Build
passed #224146
docker-alpine ci-build aarch64
build-aarch64

00:08:55

passed #224147
docker-alpine ci-build armv7
build-armv7

00:00:33

passed #224145
docker-alpine ci-build ppc64le
build-ppc64le

00:00:43

passed #224144
docker-alpine ci-build s390x
build-s390x

00:13:57

passed #224143
docker-alpine ci-build x86
build-x86

00:05:01

passed #224142
docker-alpine ci-build x86_64
build-x86_64

00:02:40

 
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failed
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