fix(download): Fix function nightly_version not defined error in scoop-download

[AkariiinMKII]

Description

Import \lib\install.ps1 in \libexec\scoop-download.ps1

Motivation and Context

Closes #6385

How Has This Been Tested?

Download a nightly build, which use "version": "nightly" in manifest
Here I'm testing with rust-nightly in versions bucket

Before changes

> scoop download rust-nightly
INFO  Downloading 'rust-nightly' [64bit] from versions bucket
nightly_version : The term 'nightly_version' is not recognized as the name of a cmdlet, function, script file, or opera
ble program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At C:\Users\A1\scoop\apps\scoop\current\libexec\scoop-download.ps1:91 char:20
+         $version = nightly_version
+                    ~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (nightly_version:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

Starting download with aria2 ...
Download: Download Results:
Download: gid   |stat|avg speed  |path/URI
Download: ======+====+===========+=======================================================
Download: 1f66a2|OK  |    29MiB/s|C:/Users/A1/scoop/cache/rust-nightly#nightly#842e246.msi
Download: Status Legend:
Download: (OK):download completed.
'rust-nightly' (nightly) was downloaded successfully!

After changes

> scoop download rust-nightly
INFO  Downloading 'rust-nightly' [64bit] from versions bucket
WARN  This is a nightly version. Downloaded files won't be verified.
Starting download with aria2 ...
Download: Download Results:
Download: gid   |stat|avg speed  |path/URI
Download: ======+====+===========+=======================================================
Download: 1326bd|OK  |    29MiB/s|C:/Users/A1/scoop/cache/rust-nightly#nightly-20250611#842e246.msi
Download: Status Legend:
Download: (OK):download completed.
'rust-nightly' (nightly-20250611) was downloaded successfully!

Checklist:

• I have read the Contributing Guide.
• I have ensured that I am targeting the develop branch.
• I have updated the documentation accordingly.
• I have updated the tests accordingly.
• I have added an entry in the CHANGELOG.

[AkariiinMKII]

@niheaven Hi, I suggest apply this fix directly with a commit to the master branch asap as the bug was imported by last update