Fix fetch-depth for Codecov reliability

[kapilt]

codecov is behaving erratically attempting to play around with cli options to address


[stefangordon]

Making note of log from last attempt to easily revisit in a bit.... Presumably some bad argument triggering the 500.

==> Reading reports
    + coverage.xml bytes=1317310
==> Appending adjustments
    https://docs.codecov.io/docs/fixing-reports
    + Found adjustments
==> Gzipping contents
        128K	/tmp/codecov.b38qu5.gz
==> Uploading reports
    url: https://codecov.io
    query: branch=releng%2Ffix-codecov&commit=50335db6cb273e125dc2420a59ee607f941f2886&build=552467963&build_url=http%3A%2F%2Fgithub.com%2Fcloud-custodian%2Fcloud-custodian%2Factions%2Fruns%2F552467963&name=&tag=&slug=cloud-custodian%2Fcloud-custodian&service=github-actions&flags=&pr=6440&job=CI&cmd_args=Z,v,f,g,U
->  Pinging Codecov
https://codecov.io/upload/v4?package=bash-20210129-7c25fce&token=secret&branch=releng%2Ffix-codecov&commit=50335db6cb273e125dc2420a59ee607f941f2886&build=552467963&build_url=http%3A%2F%2Fgithub.com%2Fcloud-custodian%2Fcloud-custodian%2Factions%2Fruns%2F552467963&name=&tag=&slug=cloud-custodian%2Fcloud-custodian&service=github-actions&flags=&pr=6440&job=CI&cmd_args=Z,v,f,g,U

==> Uploading to Codecov
    HTTP 500
Error: Process completed with exit code 1.

[kapilt]

thanks @stefangordon !