Corrected usage of shebang

[criadoperez]

Currently running ./justfile produces the following error:

$ ./justfile 
/usr/bin/env: ‘just --justfile’: No such file or directory
/usr/bin/env: use -[v]S to pass options in shebang lines

The newer versions of /usr/bin/env indicate that you should use -S to split arguments when passing them in shebang lines.
Added de required -S which fixes the problem.

 $ ./justfile
CGO_ENABLED=0 go build -trimpath -o ./bin/run-boinc ./cmd/run-boinc