Is env_var_name writable via run / export?

Build Environment
1

The following example shows

setting env_var_name Parameter dyncamically:

parameters:
      tag-or-branch:
        type: env_var_name
        default: TAGANCH

 - run:
          name: Tag or Branch
          command: |
              if [[ -z "<< pipeline.git.branch >>" ]]; then
                echo "export TAGANCH=<< pipeline.git.tag >>" >> $BASH_ENV
                else echo "export TAGANCH=<< pipeline.git.branch >>" >> $BASH_ENV
              fi

However when using the parameter, it will always contain the default value:

      - run:   
          name: Tag or Branch Debug
          command: echo "tag-or-branch ${<< parameters.tag-or-branch >>}"

will print

#!/bin/bash -eo pipefail
echo "tag_or_branch ${TAGANCH}"
tag_or_branch 
CircleCI received exit code 0

even inside the same run statement it won’ work:

  - run:
          name: Tag or Branch
          command: |
            if [[ -z "<< pipeline.git.tag >>" ]]; then
              echo "export TAGANCH=<< pipeline.git.tag >>" >> $BASH_ENV
              else echo "export TAGANCH=<< pipeline.git.branch >>" >> $BASH_ENV
            fi
            echo "tag_or_branch ${<< parameters.tag_or_branch >>}"
            echo "tag_or_branch $<< parameters.tag_or_branch >>"
            echo "tag_or_branch ${TAGANCH}"
            echo "tag_or_branch $TAGANCH"