gxpkg build should suggest `gxpkg deps -i` run first instead of the error below

[ober]

/src # gxpkg build ... build in current directory *** ERROR IN gerbil/tools/gxpkg#pkg-manifest! -- No such file or directory (call-with-input-file "/src/.gerbil/pkg/github.com/mighty-gerbils/gerbil-libyaml.manifest" '#<procedure #40 read>) --- continuation backtrace: [0] ##primordial-exception-handler-hook
[1] gerbil/tools/gxpkg#pkg-manifest!
[2] gerbil/tools/gxpkg#pkg-manifest!
[3] gerbil/tools/gxpkg#pkg-build__%