rust: pin serde version

[jasonish]

because oss-fuzz does not support newer rust nightly and newer serde does not support oss-fuzz old nightly

Make sure these boxes are checked accordingly before submitting your Pull Request -- thank you.

Contribution style:

• I have read the contributing guide lines at
  https://docs.suricata.io/en/latest/devguide/contributing/contribution-process.html

Our Contribution agreements:

• I have signed the Open Information Security Foundation contribution agreement at
  https://suricata.io/about/contribution-agreement/ (note: this is only required once)

Changes (if applicable):

• I have updated the User Guide (in doc/userguide/) to reflect the changes made
• I have updated the JSON schema (in etc/schema.json) to reflect all logging changes
  (including schema descriptions)
• I have created a ticket at
  https://redmine.openinfosecfoundation.org/projects/suricata/issues

Link to ticket: https://redmine.openinfosecfoundation.org/issues/

Describe changes:

Provide values to any of the below to override the defaults.

• To use an LibHTP, Suricata-Verify or Suricata-Update pull request,
  link to the pull request in the respective _BRANCH variable.
• Leave unused overrides blank or remove.

SV_REPO=
SV_BRANCH=
SU_REPO=
SU_BRANCH=
LIBHTP_REPO=
LIBHTP_BRANCH=