add /var/log volume mount only when EnableVarLogCollection is true

[dprotaso]

Fixes #3809

Contributes to #7881

Proposed Changes

• add /var/log volume mount only when EnableVarLogCollection is true

Release Note

We only mount a volume at /var/log if the operator has enabled log collection
Runtime contract /var/log requirement has changed from MUST to MAY

[dprotaso]

/assign @JRBANCEL

[codecov]

Codecov Report

Merging #9683 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #9683      +/-   ##
==========================================
+ Coverage   88.09%   88.12%   +0.03%     
==========================================
  Files         176      176              
  Lines        8424     8432       +8     
==========================================
+ Hits         7421     7431      +10     
+ Misses        758      757       -1     
+ Partials      245      244       -1     

Impacted Files	Coverage Δ
pkg/reconciler/revision/resources/deploy.go	100.00% <100.00%> (ø)
pkg/webhook/podspec_dryrun.go	75.86% <0.00%> (ø)
pkg/apis/config/defaults.go	91.80% <0.00%> (+0.27%)	⬆️
pkg/reconciler/configuration/configuration.go	89.16% <0.00%> (+1.66%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9076105...942aea6. Read the comment docs.

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-knative-serving-unit-tests		0/3

Failed non-flaky tests preventing automatic retry of pull-knative-serving-unit-tests:

pkg/reconciler/autoscaling/kpa.TestReconcileDeciderCreatesAndDeletes

[dprotaso]

/retest

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [dprotaso]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[mattmoor]

Produced via:
prettier --write --prose-wrap=always $(find -name '*.md' | grep -v vendor | grep -v .github | grep -v docs/cmd/)

[JRBANCEL]

/lgtm