Add comment on why MSYS2_ARG_CONV_EXCL is required

ci/scripts/PKGBUILD

@@ -80,9 +80,12 @@ build() {
   # CMAKE_UNITY_BUILD is set to OFF as otherwise some compute functionality
   # segfaults in tests
 
+  # MSYS2_ARG_CONV_EXCL is needed to prevent autoconverting CMAKE_INSTALL_PREFIX
+  # to Windows paths. See https://www.msys2.org/docs/filesystem-paths/#process-arguments
+  MSYS2_ARG_CONV_EXCL="-DCMAKE_INSTALL_PREFIX="
   # We install CMake via the msys2 GitHub action. We have to use the installed
   # CMake to build Arrow which is in the Program Files directory.
-"${PROGRAMFILES}\CMake\bin\cmake.exe" \
+  "${PROGRAMFILES}\CMake\bin\cmake.exe" \
     ${ARROW_CPP_DIR} \
     -G "MSYS Makefiles" \
     -DARROW_ACERO=ON \
@@ -129,7 +132,7 @@ package() {
   make -C ${cpp_build_dir} DESTDIR="${pkgdir}" install
 
   local PREFIX_DEPS=$(cygpath -am ${MINGW_PREFIX})
-  pushd "${pkgdir}/rtools40${MINGW_PREFIX}/lib/pkgconfig"
+  pushd "${pkgdir}${MINGW_PREFIX}/lib/pkgconfig"
   for pc in *.pc; do
     sed -s "s|${PREFIX_DEPS}|${MINGW_PREFIX}|g" -i $pc
   done