fix(dev/release): add CONDA_BUILD=1 when verifying with USE_CONDA=1 (#4180)

Ran into an issue when verifying with some driver-manager tests failing,
turned out to be the mismatch between using a conda environment but
CONDA_BUILD wasn't set. So the test for a specific error message didn't
get the message it was expecting. Adding the right `export
CONDA_BUILD=1` was sufficient to fix the verification.

Author: zeroshade

dev/release/verify-release-candidate.sh

@@ -530,6 +530,7 @@ test_cpp() {
     export CMAKE_PREFIX_PATH="${CONDA_BACKUP_CMAKE_PREFIX_PATH}:${CMAKE_PREFIX_PATH}"
     # The CMake setup forces RPATH to be the Conda prefix
     export CPP_INSTALL_PREFIX="${CONDA_PREFIX}"
+    export CONDA_BUILD=1
   else
     export CPP_INSTALL_PREFIX="${ARROW_TMPDIR}/local"
   fi