[pure] Fix bad nixpkgs breaking pure mode (#2583)

## Summary

fixes: #2532

Possibly also fixes #2535 but
need to test.

blame: #2465

cc: @Lagoja 

## How was it tested?

* unit test
* manual testing using --pure flag

# Community Contribution License

All community contributions in this pull request are licensed to the
project
maintainers under the terms of the
[Apache 2 License](https://www.apache.org/licenses/LICENSE-2.0).

By creating this pull request, I represent that I have the right to
license the
contributions to the project maintainers under the Apache 2 License as
stated in
the
[Community Contribution
License](https://github.com/jetify-com/opensource/blob/main/CONTRIBUTING.md#community-contribution-license).

Author: mikeland73

internal/devbox/devbox.go

@@ -249,7 +249,7 @@ func (d *Devbox) Shell(ctx context.Context, envOpts devopt.EnvOptions) error {
 		WithShellStartTime(telemetry.ShellStart()),
 	}
 
-	shell, err := NewDevboxShell(d, envOpts, opts...)
+	shell, err := d.newShell(envOpts, opts...)
 	if err != nil {
 		return err
 	}

internal/devbox/shell.go

@@ -69,15 +69,15 @@ type DevboxShell struct {
 
 type ShellOption func(*DevboxShell)
 
-// NewDevboxShell initializes the DevboxShell struct so it can be used to start a shell environment
+// newShell initializes the DevboxShell struct so it can be used to start a shell environment
 // for the devbox project.
-func NewDevboxShell(devbox *Devbox, envOpts devopt.EnvOptions, opts ...ShellOption) (*DevboxShell, error) {
-	shPath, err := shellPath(devbox, envOpts)
+func (d *Devbox) newShell(envOpts devopt.EnvOptions, opts ...ShellOption) (*DevboxShell, error) {
+	shPath, err := d.shellPath(envOpts)
 	if err != nil {
 		return nil, err
 	}
 	sh := initShellBinaryFields(shPath)
-	sh.devbox = devbox
+	sh.devbox = d
 
 	for _, opt := range opts {
 		opt(sh)
@@ -88,7 +88,7 @@ func NewDevboxShell(devbox *Devbox, envOpts devopt.EnvOptions, opts ...ShellOpti
 }
 
 // shellPath returns the path to a shell binary, or error if none found.
-func shellPath(devbox *Devbox, envOpts devopt.EnvOptions) (path string, err error) {
+func (d *Devbox) shellPath(envOpts devopt.EnvOptions) (path string, err error) {
 	defer func() {
 		if err != nil {
 			path = filepath.Clean(path)
@@ -110,7 +110,7 @@ func shellPath(devbox *Devbox, envOpts devopt.EnvOptions) (path string, err erro
 
 	cmd := exec.Command(
 		"nix", "eval", "--raw",
-		fmt.Sprintf("%s#bashInteractive", nix.FlakeNixpkgs(devbox.cfg.NixPkgsCommitHash())),
+		fmt.Sprintf("%s#bashInteractive", d.Lockfile().Stdenv().String()),
 	)
 	cmd.Args = append(cmd.Args, nix.ExperimentalFlags()...)
 	out, err := cmd.Output()

internal/devbox/shell_test.go

@@ -9,10 +9,12 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"go.jetify.com/devbox/internal/devbox/devopt"
 	"go.jetify.com/devbox/internal/envir"
 	"go.jetify.com/devbox/internal/shellgen"
 )
@@ -105,3 +107,61 @@ If the new shellrc is correct, you can update the golden file with:
 		})
 	}
 }
+
+func TestShellPath(t *testing.T) {
+	tests := []struct {
+		name     string
+		envOpts  devopt.EnvOptions
+		expected string
+		env      map[string]string
+	}{
+		{
+			name: "pure mode enabled",
+			envOpts: devopt.EnvOptions{
+				Pure: true,
+			},
+			expected: `^/nix/store/.*/bin/bash$`,
+		},
+		{
+			name: "pure mode disabled",
+			envOpts: devopt.EnvOptions{
+				Pure: false,
+			},
+			env: map[string]string{
+				envir.Shell: "/usr/local/bin/bash",
+			},
+			expected: "^/usr/local/bin/bash$",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			for k, v := range test.env {
+				t.Setenv(k, v)
+			}
+			tmpDir := t.TempDir()
+			err := InitConfig(tmpDir)
+			if err != nil {
+				t.Fatal("Got InitConfig error:", err)
+			}
+			d, err := Open(&devopt.Opts{
+				Dir:    tmpDir,
+				Stderr: os.Stderr,
+			})
+			if err != nil {
+				t.Fatal("Got Open error:", err)
+			}
+			gotPath, err := d.shellPath(test.envOpts)
+			if err != nil {
+				t.Fatal("Got shellPath error:", err)
+			}
+			matched, err := regexp.MatchString(test.expected, gotPath)
+			if err != nil {
+				t.Fatal("Got regexp.MatchString error:", err)
+			}
+			if !matched {
+				t.Errorf("Expected shell path %s, but got %s", test.expected, gotPath)
+			}
+		})
+	}
+}