add helpful env vars during shell + run (#1548)

## Summary

Adds a few helpful Env Vars to the shell/run/services environment. These
should help in situations where a developer needs to refer to files
within their project directory, but doesn't want to hardcode the path
into their configuration

Since this just adds a few variables to the shell environment, it should
be a fairly low risk change

## How was it tested?

Locally

Author: Lagoja

internal/impl/devbox.go

@@ -766,6 +766,7 @@ func (d *Devbox) computeNixEnv(ctx context.Context, usePrintDevEnvCache bool) (m
 	// Append variables from current env if --pure is not passed
 	currentEnv := os.Environ()
 	env, err := d.parseEnvAndExcludeSpecialCases(currentEnv)
+
 	if err != nil {
 		return nil, err
 	}
@@ -869,6 +870,11 @@ func (d *Devbox) computeNixEnv(ctx context.Context, usePrintDevEnvCache bool) (m
 		return nil, err
 	}
 
+	// Add helpful env vars for a Devbox project
+	env["DEVBOX_PROJECT_ROOT"] = d.projectDir
+	env["DEVBOX_CONFIG_DIR"] = d.projectDir + "/devbox.d"
+	env["DEVBOX_PACKAGES_DIR"] = d.projectDir + "/.devbox/virtenv/.wrappers"
+
 	// Include env variables in devbox.json
 	configEnv, err := d.configEnvs(ctx, env)
 	if err != nil {