ARROW-12406: [R] Fix checkbashism violation in configure

Fix checkbashim foo=bar; export foo

Closes #10052 from jonkeane/ARROW-12406

Authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>
diff --git a/r/configure b/r/configure
index eea7af6..f6c1013 100755
--- a/r/configure
+++ b/r/configure
@@ -148,7 +148,7 @@
       if [ "$UNAME" = "Darwin" ] && [ "${OPENSSL_ROOT_DIR}" = "" ]; then
         brew --prefix openssl >/dev/null 2>&1
         if [ $? -eq 0 ]; then
-          export OPENSSL_ROOT_DIR="$(brew --prefix openssl)"
+          OPENSSL_ROOT_DIR="`brew --prefix openssl`"; export OPENSSL_ROOT_DIR
         fi
       fi