GH-47534: [C++] Detect conda-installed packages in Meson CI (#47535)

### Rationale for this change

This detects conda-installed packages in Meson CI, so that dependencies which are installed via conda do not need to be built from scratch

### What changes are included in this PR?

An update to the meson setup invocation

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* GitHub Issue: #47534

Authored-by: Will Ayd <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: WillAyd

ci/scripts/cpp_build.sh

@@ -142,6 +142,7 @@ if [ "${ARROW_USE_MESON:-OFF}" = "ON" ]; then
   meson setup \
     --prefix=${MESON_PREFIX:-${ARROW_HOME}} \
     --buildtype=${ARROW_BUILD_TYPE:-debug} \
+    --pkg-config-path="${CONDA_PREFIX}/lib/pkgconfig/" \
     -Dauto_features=enabled \
     -Dfuzzing=disabled \
     -Dgcs=disabled \