tfa-next(next-checks-clippy): fix available platforms command

When calling `make -C rust list_platforms` to get the available
platforms of tfa-next, currently we are also saving the information
about make changing the directory and this gets confused with a new
platform to run clippy on.

 * Use make's --silent to silence the cd command and only get the
   listed platforms.

Change-Id: Ib8bb36c553e602837ec66770c65ba58c99e90a53
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
diff --git a/script/next-checks/next-checks-clippy.sh b/script/next-checks/next-checks-clippy.sh
index 4b07b33..5b8539d 100755
--- a/script/next-checks/next-checks-clippy.sh
+++ b/script/next-checks/next-checks-clippy.sh
@@ -19,7 +19,7 @@
 echo >> "$LOG_TEST_FILENAME"
 echo "Platforms:" >> "$LOG_TEST_FILENAME"
 
-available_platforms=$(make -C ${TF_ROOT}/rust list_platforms)
+available_platforms=$(make --silent -C ${TF_ROOT}/rust list_platforms)
 
 for plat in $available_platforms
 do