Mount shards in `/opt/triplet(p)` instead of `/opt/aatriplet(p)`

[giordano]

In

BinaryBuilderBase.jl/src/Rootfs.jl

Lines 164 to 166 in c67ccbe

	return "/opt/$(aatriplet(cs.host))/$(cs.name)-$(cs.version)-$(aatriplet(cs.target)))"
	else
	return joinpath("/opt", aatriplet(something(cs.target, cs.host)), "$(cs.name)-$(cs.version)")

we eventually want to mount the compiler shards on a directory which contains the full triplet, not the ABI-agnostic one. Once #83 is resolved, this will allow use to effectively use two different compilers for the host (x86_64-linux-musl-cxx11, GCC v5+) and the target also when the target is x86_64-linux-musl-cxx03 with preferred GCC v4