Remove unneeded Invoke-Expression on unvalidated input - #8826

PR Summary

Remove unneeded Invoke-Expression on unvalidated input

PR Context

Reviewing the script for security issue that might prevent signing as requested in #8814

I didn't validate that this could actually be exploited but I ran this.

$tp=join-path "./;Write-Host 'pwnd';" -ChildPath 'child'
Invoke-Expression -command "ls $tp"

With this output

CHANGELOG.md Dockerfile Library bin manpages
CODE_OF_CONDUCT.md Dockerfile.test.yml README.md completions
CONTRIBUTING.md LICENSE.txt azure-pipelines.yml docs
pwnd
/child : The term '/child' is not recognized as the name of a cmdlet, function, script file, or operable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:25
+ ls ./;Write-Host 'pwnd';/child
+ ~~~~~~
+ CategoryInfo : ObjectNotFound: (/child:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

While the equivalent new syntax:

&ls $tp

results in:

ls: ./;Write-Host 'pwnd';/child: No such file or directory

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