Unable to build the Swift toolchain with swiftly

[MaxDesiatov]

As pointed out by a contributor, when swiftly is installed and selected, Swift toolchain fails to build:

ninja: error: '/home/rup/.local/share/swiftly/lib/swift/linux/x86_64/swiftrt.o', needed by 'lib/libswiftDemangle.so', missing and no known rule to make it

[cmcgee1024]

With #321 I hope that this kind of occurrence should be reduced since swiftly's bin dir will no longer be on the path after proxying, or swift run the build script.