If we don't quote it, paths containing spaces or quote characters will
fail. For instance, the added test without the fix fails:
> env NAME='with spaces'
> mkdir $NAME
> cp $EXEC_PATH $NAME/garble$exe
> exec $NAME/garble$exe build main.go
[stderr]
go tool compile: fork/exec $WORK/with: no such file or directory
exit status 1
Luckily, the fix is easy: we bundle Go's cmd/internal/quoted package,
which implements a QuotedJoin API for this very purpose.
Fixes#544.
Daniel Martí