ci: simulate Pikaur flag duplication in Arch Linux CI

Inject system flags from /etc/makepkg.conf into the CI environment to
detect build collisions caused by Pikaur's configuration bug.

Pikaur's cascading logic causes flags from /etc/makepkg.conf to be
merged into the build environment. This creates collisions with flags
defined in the project's Makefile.am (e.g., duplicate -D_FORTIFY_SOURCE
definitions), which can cause builds to fail for users.

By exporting these flags in the CI environment, we ensure that any
code change that is sensitive to flag duplication will trigger a
failure in our Arch Linux CI matrix, preventing broken builds from
reaching users.

Implementation details:
- Detects Arch Linux via /etc/os-release.
- Uses a sed-based flattener to handle multi-line variables and
  trailing backslashes in makepkg.conf.
- Exports the flags to the shell environment so that 'configure'
  and 'make' inherit them naturally, maintaining parity with a
  real Pikaur session.

ci-build.sh

@@ -204,6 +204,13 @@ case "$ARCH" in
         ""
         )
         source /etc/profile.d/debuginfod.sh 2>/dev/null || true
+
+        if grep -q 'ID=arch' /etc/os-release 2>/dev/null && [ -f /etc/makepkg.conf ]; then
+            echo "--> [Parity Mode] Simulating Pikaur collision..."
+            set -a
+            source /etc/makepkg.conf
+            set +a
+        fi
         ;;
     darwin*)
         # 4 configurations for parallel CI