ci: Make rust build nonblocking for now

Will debug at some point but for now let's
unblock other things.

```
/usr/bin/ld: /var/tmp/checkout/target/release/libbupsplit_rs.a(bupsplit_rs-db7d02fa07221ce3.bupsplit_rs0.rust-cgu.o): undefined reference to symbol 'dladdr@@GLIBC_2.2.5'
```

Closes: #1387
Approved by: jlebon

.papr.yml

@@ -53,6 +53,7 @@ tests:
 
 context: f26-rust
 inherit: true
+required: false
 container:
     image: registry.fedoraproject.org/fedora:26
 env:
@@ -67,6 +68,7 @@ tests:
 
 context: f26-gnutls
 inherit: true
+required: true
 container:
     image: registry.fedoraproject.org/fedora:26
 env: