Don't request backend offers if creation_policy: reuse or instances is set

[un-def]

Previously, they were collected unconditionally only to be immediately discarded

[un-def]

Before:

time dstack offer --reuse --cpu 0.. --memory 0.. --disk 0.. --gpu 0..
dstack offer  1.06s user 0.07s system 18% cpu 6.185 total

DEBUG    dstack._internal.server.services.backends:492 Requesting instance offers from backends: ['aws', 'runpod', 'gcp']
DEBUG    dstack._internal.server.services.backends:529 Got offers from runpod in 0.091026s
DEBUG    dstack._internal.server.services.backends:529 Got offers from gcp in 1.963004s
DEBUG    dstack._internal.server.services.backends:529 Got offers from aws in 2.918598s

After:

time dstack offer --reuse --cpu 0.. --memory 0.. --disk 0.. --gpu 0..
dstack offer --reuse  1.02s user 0.07s system 97% cpu 1.117 total