~ CI: install xsltproc on Linux (test_SPARQL GRDDL0 shells out to it)

macOS ships xsltproc; the ubuntu runner does not, which surfaced once
test_SPARQL joined the blocking set.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Eric Prud'hommeaux

.github/workflows/ci.yml

@@ -23,7 +23,8 @@ jobs:
             libboost-dev libboost-regex-dev libboost-thread-dev \
             libboost-filesystem-dev libboost-date-time-dev \
             libboost-program-options-dev libboost-iostreams-dev \
-            libboost-log-dev libboost-test-dev
+            libboost-log-dev libboost-test-dev \
+            xsltproc
 
       - name: Install dependencies (macOS)
         if: runner.os == 'macOS'