cr50 signer: fix file name variable
cr50 signer: fix file name variable

There is no need to add hardcoded .test suffix when determining the
base RMA key file name.

BRANCH=none
BUG=none
TEST=succeeded signing both prod and pre-pvt images.

Change-Id: I59a5eb4ff8c093110c4d29969974148c99bd62a0
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1099731
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec