fix: don't assume the login challenge to be a UUID

[hperl]

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #3317 (7d1e3d4) into master (3485204) will decrease coverage by 0.08%.
The diff coverage is 91.11%.

❗ Current head 7d1e3d4 differs from pull request most recent head 5871f81. Consider uploading reports for the commit 5871f81 to get more accurate results

@@            Coverage Diff             @@
##           master    #3317      +/-   ##
==========================================
- Coverage   78.20%   78.12%   -0.08%     
==========================================
  Files         325      325              
  Lines       21079    21080       +1     
==========================================
- Hits        16484    16469      -15     
- Misses       3370     3383      +13     
- Partials     1225     1228       +3     

Impacted Files	Coverage Δ
selfservice/flow/registration/flow.go	97.36% <ø> (ø)
hydra/fake.go	70.00% <66.66%> (ø)
hydra/hydra.go	67.10% <92.30%> (-0.43%)	⬇️
selfservice/flow/login/handler.go	79.38% <93.75%> (+0.18%)	⬆️
selfservice/flow/login/flow.go	95.00% <100.00%> (ø)
selfservice/flow/login/hook.go	86.23% <100.00%> (ø)
selfservice/flow/registration/handler.go	78.78% <100.00%> (ø)
selfservice/flow/registration/hook.go	70.92% <100.00%> (ø)

... and 2 files with indirect coverage changes

[aeneasr]

Looking very good. You could improve the SQL migration, right now it will probably cause some 500s when migrating.