You have to copy the file plutus-pioneer-program/code/week01/src/Week01/EnglishAuction.hs manually and paste it in the playground running in the Browser(you can delete the text in an example contract that is in the playground already and paste it in its place) and compile.
For example one handy cli method in 2 steps is
step-1: xclip -i plutus-pioneer-program/code/week01/src/Week01/EnglishAuction.hs
step-2: simultaneous right and left mouseclick to paste and erase from clipboard.(Paste in the playground running in the browser)
cabal build command is useful so that you can run cabal repl from the terminal for an interactive session with the auction contract.