printers: Fix GCancellable leak

Use the dialog cancellable instead of leaking one each time the PPD is set.
11 jobs for <span class="ref-name">printers-details-dialog-cancellables</span> in 3 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #199954
build

00:01:50

 
  Test
passed #199956
asan

00:01:37

passed #199955
test

00:01:26

 
  Manual
manual #199961
allowed to fail manual
armv8
manual #199957
allowed to fail manual
flatpak
manual #199960
allowed to fail manual
msan
manual #199962
allowed to fail manual
ppc64le
manual #199958
allowed to fail manual
tsan
manual #199959
allowed to fail manual
ubsan
 
  Review
manual #199963
allowed to fail manual
review
manual #199964
allowed to fail manual
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