ci: add architecture specification to agent label

Should fix errors like:
```
clang: error: the clang compiler does not support '-march=native'
```
Caused by hitting the Apple M1 `arm64` host.

Signed-off-by: Jakub Sokołowski <[email protected]>

Author: jakubgs

ci/Jenkinsfile.prs

@@ -1,7 +1,7 @@
 library '[email protected]'
 
 pipeline {
-  agent { label getAgentLabel() }
+  agent { label "${getAgentLabel()} && x86_64" }
 
   parameters {
     string(