[per-OS Packages] Suggest --platform/--exclude-platform upon error with installing package that is uninstallable on user's platform (#1364)

## Summary

Changes the error message to suggest using
`--platform/--exclude-platform` with `devbox add`.

I wish we had a way to recommend the platforms that are supported for
the exact package being installed, but we don't.

The heavy lifting for this was done in #1279. Thanks @mikeland73!

## How was it tested?

```
> devbox add glibcLocales

Installing package: glibcLocales.


Error: package glibcLocales cannot be installed on your platform x86_64-darwin. Consider using `--platform` or `--exclude-platform` with `devbox add` to install on a supported platform. Other available platforms are: aarch64-darwin, aarch64-linux, i686-linux, x86_64-linux, armv7l-linux.
```

<img width="949" alt="Screenshot 2023-08-10 at 4 20 18 PM"
src="https://github.com/jetpack-io/devbox/assets/676452/95ddff88-793b-4c09-9e65-be344d303c2a">

Author: savil

internal/nix/nix.go

@@ -195,7 +195,7 @@ var nixPlatforms = []string{
 	"armv7l-linux",
 }
 
-// ensureValidPlatform returns an error if the platform is not supported by nix.
+// EnsureValidPlatform returns an error if the platform is not supported by nix.
 // https://nixos.org/manual/nix/stable/installation/supported-platforms.html
 func EnsureValidPlatform(platforms ...string) error {
 	ensureValid := func(platform string) error {

internal/nix/nixprofile/profile.go

@@ -265,9 +265,17 @@ func ProfileInstall(args *ProfileInstallArgs) error {
 		if exists, err := input.ValidateInstallsOnSystem(); err != nil {
 			return err
 		} else if !exists {
+			platform, err := nix.System()
+			if err != nil {
+				platform = ""
+			} else {
+				platform = " " + platform
+			}
 			return usererr.New(
-				"package %s cannot be installed on your system. It may be installable on other systems.",
-				input.String(),
+				"package %s cannot be installed on your platform%s. "+
+					"Run `devbox add %[1]s --exclude-platform%[2]s` and re-try.",
+				input.Raw,
+				platform,
 			)
 		}
 	}