devcontainer: allow BUILDROOT_OUTPUT to be overridden

[colemancda]

swift-define computed BUILDROOT_OUTPUT unconditionally from WORKING_DIR and SWIFT_TARGET_ARCH, so a caller whose output tree lives elsewhere could not use install-swift.sh at all.

The toolchain CI builds into a persistent directory on the self-hosted runner's own disk (O=/mnt/br/shared), which no combination of those two variables can spell. This gives BUILDROOT_OUTPUT the same "${VAR:=default}" treatment the other configurables already have — the existing layout stays the default.

Why

This unblocks using the prebuilt Swift toolchain in CI instead of building host-swift from source, which takes 5–6 hours and has now failed four separate ways (wasi-libc pin mismatch, stale-checkout API mismatch, -index-store-path on the distro clang, and scattered CMake cache corruption from resumed builds).

Verified end-to-end inside colemancda/buildroot-swift:latest:

$ BUILDROOT_OUTPUT=/out SWIFT_BUILDROOT=/tmp/ext bash .devcontainer/library-scripts/install-swift.sh
Download https://download.swift.org/swift-6.3.3-release/debian12/swift-6.3.3-RELEASE/swift-6.3.3-RELEASE-debian12.tar.gz
Install LLVM

$ swiftc --version
Swift version 6.3.3 (swift-6.3.3-RELEASE)

$ clang --version
clang version 21.0.0 (https://github.com/swiftlang/llvm-project.git 82cdc19fa54d566969527b56f587ea8ea30bef51)

$ clang -index-store-path /tmp/idx -c t.c -o t.o   # the flag that broke CI
$ echo $?
0

The prebuilt clang is the same swiftlang/llvm-project commit the source build was producing, and it supports -index-store-path — so it also resolves that failure directly.

HOST_SWIFT_BUILD_CMDS is already guarded by if [ ! -d "$(SWIFT_LLVM_DIR)" ], so populating build/usr and build/llvm makes build-script skip entirely; no change to swift.mk is needed.