Detect CI builds for every CI system

In order to unify the environment variable to detect whether a build is a CI build as simply CI (as is done already in GitHub Actions, Circle CI and GitLab), you just place this bit of MSBuild in every project's Directory.Build.props to make things consistent everywhere:

<PropertyGroup Label="CI" Condition="'$(CI)' == ''">
<!-- GH, CircleCI, GitLab and BitBucket already use CI -->
<CI Condition="'$(TF_BUILD)' == 'true' or
'$(TEAMCITY_VERSION)' != '' or
'$(APPVEYOR)' != '' or
'$(BuildRunner)' == 'MyGet' or
'$(JENKINS_URL)' != '' or
'$(TRAVIS)' == 'true' or
'$(BUDDY)' == 'true'">true</CI>