script: test-repro-build: change imx8 with AMD's rmb

XTOS is going to be deprecated on all NXP's platforms so
replace imx8 with AMD's rmb so that the CI test can be kept.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>

scripts/test-repro-build.sh

@@ -39,7 +39,7 @@ SOF2="$SOF_PARENT"/sof-bind-mount-DO-NOT-DELETE
 # considerably, replace "sof" with something of the same length:
 # SOF2="$SOF_PARENT"/sog
 
-PLATFS=(imx8)
+PLATFS=(rmb)
 
 # diffoscope is great but it has hundreds of dependencies, too long to
 # install for CI so we don't use it here. This is just an alias