fix specs_prefix_format to point to root toolchain's directory #126

Job	Run time
cache-maker	6m 8s
minsize-zephyr (./.github/do-zephyr)	36m 40s
release-zephyr (./.github/do-zephyr)	37m 38s
minsize-zephyr (-Dio-float-exact=false -Dio-long-long=true -Dio-percent-b=true -Dio-long-double=t...	35m 50s
release-zephyr (-Dio-float-exact=false -Dio-long-long=true -Dio-percent-b=true -Dio-long-double=t...	36m 22s
minsize-zephyr (-Dformat-default=integer -Dfreestanding=true -Dposix-io=false -Dnewlib-obsolete-m...	35m 34s
release-zephyr (-Dformat-default=integer -Dfreestanding=true -Dposix-io=false -Dnewlib-obsolete-m...	36m 59s
minsize-zephyr (-Dtinystdio=false, ./.github/do-zephyr)	20m 17s
release-zephyr (-Dtinystdio=false, ./.github/do-zephyr)	21m 31s
minsize-zephyr (-Dtinystdio=false -Dnewlib-io-float=true -Dio-long-long=true -Dio-long-double=tru...	17m 59s
release-zephyr (-Dtinystdio=false -Dnewlib-io-float=true -Dio-long-long=true -Dio-long-double=tru...	18m 44s
	5h 3m 42s

Provide feedback