.\build.ps1 in its current state isn't capable for :add-package command

### Usage Information

Fallout 10.4.0.15 (Global Tool)

### Description

Running `fallout :add-package Package --version x.y.z` does not work anymore.

The problem is the fancy "Join-Path ..." in the ps1 bootstrapper script.
This causes the line 35-36 of `Fallout.Cli.AddPackageCommand` to give this as build script path: `Join-Path C:\dev\RootDir 'build/_build.csproj'` which is utterly wrong...

### Reproduction Steps

- Run `fallout :add-package GitVersion.Tool --version 6.8.2` on a solution with Fallout installed
- (needs the line `$BuildProjectFile = Join-Path "$PSScriptRoot" 'build/_build.csproj'` in build.ps1, otherwise it fails anyway with a "KeyNotFoundException)

### Expected Behavior

Adds the package

### Actual Behavior

Gives a powershell only command back

### Regression?

Yes. Works on solutions which were set up with NUKE, which have this instead of "Join-Path": `$BuildProjectFile = "$PSScriptRoot\build\_build.csproj"`

### Known Workarounds

Restoring this exact string

### Could you help with a pull-request?

Yes