[Fix] Ensure python sys path contains nix installed modules

[mikeland73]

Summary

Fixes #216

Fixes issue where python can't find modules installed as nix packages.

How was it tested?

testdata/python/pipenv

devbox shell
python -m pipenv

[savil]

hmm the test failure looks legit...

--- FAIL: TestDevbox (0.16s)
    --- FAIL: TestDevbox/testdata/python/pipenv (0.00s)
        devbox_test.go:58:
            	Error Trace:	/Users/savil/code/jetpack/devbox/devbox_test.go:58
            	Error:      	Received unexpected error:
            	            	Devbox could not find a buildable plan for this project. If your project/language is currently supported, please create an issue at https://github.com/jetpack-io/devbox/issues - if it's not supported you can request it!
            	            	go.jetpack.io/devbox/boxcli/usererr.New
            	            		/Users/savil/code/jetpack/devbox/boxcli/usererr/usererr.go:15
            	            	go.jetpack.io/devbox/planner.GetBuildPlan
            	            		/Users/savil/code/jetpack/devbox/planner/planner.go:112
            	            	go.jetpack.io/devbox.(*Devbox).BuildPlan
            	            		/Users/savil/code/jetpack/devbox/devbox.go:151
            	            	go.jetpack.io/devbox.(*Devbox).generateBuildFiles
            	            		/Users/savil/code/jetpack/devbox/devbox.go:246
            	            	go.jetpack.io/devbox.(*Devbox).Generate
            	            		/Users/savil/code/jetpack/devbox/devbox.go:164
            	            	go.jetpack.io/devbox.testExample.func1
            	            		/Users/savil/code/jetpack/devbox/devbox_test.go:56
            	            	testing.tRunner
            	            		/usr/local/go/src/testing/testing.go:1446
            	            	runtime.goexit
            	            		/usr/local/go/src/runtime/asm_amd64.s:1594
            	            	go.jetpack.io/devbox.(*Devbox).generateBuildFiles
            	            		/Users/savil/code/jetpack/devbox/devbox.go:248
            	            	go.jetpack.io/devbox.(*Devbox).Generate
            	            		/Users/savil/code/jetpack/devbox/devbox.go:164
            	            	go.jetpack.io/devbox.testExample.func1
            	            		/Users/savil/code/jetpack/devbox/devbox_test.go:56
            	            	testing.tRunner
            	            		/usr/local/go/src/testing/testing.go:1446
            	            		....

[savil]

sent #219 to fix test