CI: Disable sandboxing in fsanitize=address,undefined job.

The sandboxing on Linux now supports Landlock, which restricts all
supported filesystem actions after xz opens the files it needs. The
sandbox is only enabled when one file is input and we are writing to
standard out. With fsanitize=address,undefined, the instrumentation
needs to read additional files after the sandbox is in place. This
forces all xz based test to fail, so the sandbox must instead be
disabled.

.github/workflows/ci.yml

@@ -80,13 +80,17 @@ jobs:
 
       # ifunc must be disabled for this test because __attribute__ ifunc is
       # incompatible with -fsanitize=address.
+      #
+      # The sandbox must also be disabled because it will prevent access to
+      # the /proc/ filesystem on Linux, which is used by the sanitizer's
+      # instrumentation.
       - name: Build with -fsanitize=address,undefined
         if: ${{ matrix.os == 'ubuntu-latest' && matrix.build_system == 'autotools' }}
-        run: ./build-aux/ci_build.sh -b autotools -p build -f "-fsanitize=address,undefined" -d ifunc
+        run: ./build-aux/ci_build.sh -b autotools -p build -f "-fsanitize=address,undefined" -d ifunc,sandbox
       - name: Test with -fsanitize=address,undefined
         if: ${{ matrix.os == 'ubuntu-latest' && matrix.build_system == 'autotools' }}
         run: |
-            ./build-aux/ci_build.sh -b autotools -p test -f "-fsanitize=address,undefined" -d ifunc
+            ./build-aux/ci_build.sh -b autotools -p test -f "-fsanitize=address,undefined" -d ifunc,sandbox
             cd ../xz_build && make distclean
 
       - name: Build with full features