community/darktable: build with -O2 to fix build

Building with -Os segfaults 3.6.0, and as this is a computation heavy
application, building it with -O2 makes sense, while not increasing the
package size all that much:

> Size difference for darktable: 15 MiB -> 16 MiB
8 jobs for !23682 with darktable-fix-build in 1 minute and 57 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Lint
passed #448942
docker-alpine x86_64
lint

00:00:20

 
  Build
passed #448947
docker-alpine ci-build aarch64
build-aarch64

00:00:27

passed #448949
docker-alpine ci-build armhf
build-armhf

00:00:29

passed #448948
docker-alpine ci-build armv7
build-armv7

00:00:25

passed #448946
docker-alpine ci-build ppc64le
build-ppc64le

00:00:49

passed #448945
docker-alpine ci-build s390x
build-s390x

00:00:27

passed #448944
docker-alpine ci-build x86
build-x86

00:00:14

passed #448943
docker-alpine ci-build x86_64
build-x86_64

00:00:15