[allow unfree] add --impure and env-var to nix profile commands (#753)

## Summary

Adding these back. They enable adding unfree packages and do not affect
shell perf.

```
❯ time devbox run -- echo "hi"
Warning: local and global devbox.json have different nixpkgs commits. Will use the local version. This may lead to version mismatch and nix store bloat.
Ensuring packages are installed.
hi
devbox run -- echo "hi"  0.41s user 0.16s system 67% cpu 0.847 total
```

context:
#749 (comment)

## How was it tested?

these now work
```
> devbox add slack
> devbox add vscode
```

~I will add a testscript unit test in a follow up PR~

Added testscript unit test

Author: savil

examples/testdata/unfree-packages/devbox.json

@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"os"
 	"os/exec"
 	"strconv"
 	"strings"
@@ -32,11 +33,6 @@ func ProfileListItems(writer io.Writer, profileDir string) ([]*NixProfileListIte
 	// that we need to parse.
 	cmd.Stderr = writer
 
-	//out, err := cmd.Output()
-	//if err != nil {
-	//	return nil, errors.WithStack(err)
-	//}
-
 	// The `out` output is of the form:
 	// <index> <UnlockedReference> <LockedReference> <NixStorePath>
 	//
@@ -230,8 +226,10 @@ func profileInstall(args *ProfileInstallArgs) error {
 	cmd := exec.Command(
 		"nix", "profile", "install",
 		"--profile", args.ProfilePath,
+		"--impure", // for NIXPKGS_ALLOW_UNFREE
 		FlakeNixpkgs(args.NixpkgsCommit)+"#"+args.Package,
 	)
+	cmd.Env = AllowUnfreeEnv()
 	cmd.Args = append(cmd.Args, ExperimentalFlags()...)
 	if args.Priority != "" {
 		cmd.Args = append(cmd.Args, "--priority", args.Priority)
@@ -300,8 +298,10 @@ func ProfileRemove(profilePath, nixpkgsCommit, pkg string) error {
 	}
 	cmd := exec.Command("nix", "profile", "remove",
 		"--profile", profilePath,
+		"--impure", // for NIXPKGS_ALLOW_UNFREE
 		info.attributeKey,
 	)
+	cmd.Env = AllowUnfreeEnv()
 	cmd.Args = append(cmd.Args, ExperimentalFlags()...)
 	out, err := cmd.CombinedOutput()
 	if bytes.Contains(out, []byte("does not match any packages")) {
@@ -310,3 +310,7 @@ func ProfileRemove(profilePath, nixpkgsCommit, pkg string) error {
 
 	return errors.Wrap(err, string(out))
 }
+
+func AllowUnfreeEnv() []string {
+	return append(os.Environ(), "NIXPKGS_ALLOW_UNFREE=1")
+}

internal/nix/profile.go

@@ -0,0 +1,11 @@
+# Test ensures that we can add and remove "unfree" nix packages
+
+exec devbox init
+
+# we could test with slack and/or vscode. Using slack since it is lighter.
+exec devbox add slack
+stderr 'Installing package: slack.'
+stderr 'is now installed.'
+
+exec devbox rm slack
+stderr 'is now removed.'