cherry-pick: Add `/var/log` mount to log agent (#4413) #15813

Job	Run time
Check generated files	46s
Run linting	1m 21s
Run helm unit tests	7s
Run helm linting	4s
Code security scanning alerts	28s
Run unit tests	2m 43s
Lint markdown files	13s
Prepare properties	4s
Build images (amd64)	3m 8s
Build images (arm64)	4s
Build images (ppc64le)	4s
Build images (s390x)	4s
Push images (amd64)	22s
Push images (arm64)	4s
Push images (ppc64le)	7s
Push images (s390x)	4s
Create manifest	19s
	10m 2s

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