[ issue 1246 ] Fixed by using the tag v0.8 from the standard library

The fix requires running `make up-to-date-std-lib` before running
`make library-test` when changing between the master and the
maintenance branches.

N.B. After this commit, the maint-2.4.0 branch is 1 commit ahead from
the master branch, so the maint-2.4.0 branch shouldn't be merged in
the master branch.
[ tiny refactor ] A use of addContext.
Merge branch 'maint-2.4.0' of github.com:agda/agda into maint-2.4.0
Merge branch 'maint-2.4.0'
[ merge maint ] restored Makefile goal up-to-date-std-lib for master.