secret-store: Use protocol attribute for lookup

Without being able to rely on the schema name for the lookup,
we need a different attribute.

This is a hack we can use as long as there is only one protocol (or we would
need to call this multiple times for each protocol, which would also be doable).

With the last commit and without this one we get warnings from chatty:

`WARNING: secret_service_search: must specify at least one attribute to match`
7 jobs for fix-secret-store in 4 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #359675
librem5
build-pureos-gcc-byzantium

00:01:51

 
  Test
passed #359677
librem5
check-po

00:00:19

passed #359676
librem5
test:debian-gcc

00:02:00

23.2%
 
  Package
canceled #359678
librem5
package-pureos-byzantium

00:00:59

canceled #359679
librem5:arm64
package-pureos-byzantium:arm64

00:01:01

 
  Test Package
canceled #359680
librem5
autopkgtest-pureos-byzantium-package

canceled #359681
librem5
lintian-pureos-byzantium-package