granted: fix wrapping of assume script

This fixes a few problems with the assume script in the granted package.

- assume is currently being exec'ed by a wrapper script created with
  wrapProgram, but it should be sourced. This causes the error:
```
line 172: return: can only `return' from a function or sourced script
```

- assume inspects argv[0] to check if an alias has been added to the
  user's shell profile. This means a wrapper of any kind won't work,
  even if it called source instead of exec, because source will still
  set argv[0]. This means assume always tries to add the alias, but it
  fails when it has already added the alias.
```
[i] To assume roles with Granted, we need to add an alias to your shell profile (https://docs.commonfate.io/granted/internals/shell-alias)
? Install zsh alias at /Users/matthew/.zshenv Yes

[✘] the Granted alias has already been added to /Users/matthew/.zshenv
```

- assume depends on assumego, so if it is run directly from e.g. a
  result directory created with `nix build`, it will output
```
line 22: assumego: command not found
```

Fix all of these problems by dropping the wrapper around assume, and
instead insert a few lines into the start of the assume script that add
xdg-utils and granted to assume's PATH.

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