You only need to use “&” to concatenate when you are inside of a relevance substitution; otherwise just use {}.
So this line I think should read
parameter "PSFile" = "C:\windows\temp\BigFix_deployScripts\{parameter "contentInstallation"}Install.ps1"
And later I think
new-item -itemtype file -path C:\windows\temp\BigFix_deploy&{parameter “contentInstallation”}&found.InstallSuccess
should instead read
new-item -itemtype file -path C:\windows\temp\BigFix_deploy\{parameter "contentInstallation"}found.InstallSuccess