Please show exactly how the step is in Jenkinsfile.

I am attaching the Jenkins file here I am also unable to read the parameters given in the email in the post section
Test_env.jenkinsfile

You use triple apostrophes '''

                            bat'''
                                echo "The build number is ${env.BUILD_NUMBER}"
                                (
                                    echo "The build number is ${env.BUILD_NUMBER}"
                                )>build.txt
                                '''

Change them to """ and then it should work.

See https://groovy-lang.org/syntax.html#_triple_single_quoted_string and https://groovy-lang.org/syntax.html#_triple_double_quoted_string.

Is there a way to print all the environment variables to a list or dictionary, So we can add to influx as in

script {
    def my_custom_data = [:]
    my_custom_data["branch"] = env.Branch    // or whatever the env variable is called
    my_custom_data["changeset"] = env.Changeset    // or whatever the env variable is called
    // ...
    influxDbPublisher(selectedTarget: "my-target", customData: my_custom_data)
}

I don't know. The env global variable seems to be implemented in EnvActionImpl.java, where the getProperty method returns the value of the requested environment variable. So it's not a normal Map<String, String>. I don't know whether it is possible to enumerate its keys and values in Groovy. It has the getOverriddenEnvironment method that returns such a map, but it might not be possible to call that method from Groovy.