ci: move test environment variable declaration to .cirrus.yml

environment var moved:
    1. SECP256K1_TEST_ITERS (replaces TEST_ITERS)
    2. SECP256K1_BENCH_ITERS (replaces BENCH_ITERS)

ci/cirrus.sh

@@ -32,10 +32,6 @@ file .libs/* || true
 # This tells `make check` to wrap test invocations.
 export LOG_COMPILER="$WRAPPER_CMD"
 
-# This limits the iterations in the tests and benchmarks.
-export SECP256K1_TEST_ITERS="$TEST_ITERS"
-export SECP256K1_BENCH_ITERS="$BENCH_ITERS"
-
 make "$BUILD"
 
 if [ "$BENCH" = "yes" ]