Bump IBEX_COSIM_VERSION to pick up new spike version in CI

Signed-off-by: Harry Callahan <hcallahan@lowrisc.org>

Author: hcallahan-lowrisc

ci/vars.env

@@ -6,7 +6,7 @@
 # Quote values to ensure they are parsed as string (version numbers might
 # end up as float otherwise).
 VERILATOR_VERSION=v4.210
-IBEX_COSIM_VERSION=39612f9
+IBEX_COSIM_VERSION=6d5b660
 RISCV_TOOLCHAIN_TAR_VERSION=20220210-1
 RISCV_TOOLCHAIN_TAR_VARIANT=lowrisc-toolchain-gcc-rv32imcb
 RISCV_COMPLIANCE_GIT_VERSION=844c6660ef3f0d9b96957991109dfd80cc4938e2