[tryhaskell] We should use cabal exec

[relrod]

00:42:03 < mietek> Instead of mucking around with GHC_PACKAGE_PATH manually, all that is needed is
00:42:03 < mietek> cabal --sandbox-config-file=/app/sandbox/.halcyon-sandbox.config exec -- mueval -e '1+1'
00:42:04 < mietek> (or use CABAL_SANDBOX_CONFIG)
00:44:14 < mietek> haskell/cabal#1944 (comment)

Thanks @mietek

[mietek]

I see in the playbooks here the sandbox for tryhaskell is created in /srv/web/tryhaskell. Therefore, in this config, it should be enough to cd /srv/web/tryhaskell && cabal exec -- mueval -e '1+1'.