CI: Run Linux arm64 tests only once a week on arm1.small instance at AWS CodeBuild

[tatsuya6502]

Related to #442.

For the record, I pushed this commit 36b49b4 to the main branch directly by mistake. I meant to push it to a topic branch and create a pull request.

The commit changed Linux arm64 tests as the followings to utilize the free tier of AWS CodeBuild:

• Run the job only once a week by cron schedule.
  • Before this change, it was triggered on every git push.
• Change the EC2 instance from arm1.medium (4 vCPUs, 8 GB RAM) to arm1.small (2 vCPUs, 3 GB RAM).

Pricing information of CodeBuild

• https://aws.amazon.com/codebuild/pricing/

Free Tier
The AWS CodeBuild free tier includes 100 total build minutes per month with the general1.small or arm1.small instance types using on-demand EC2. ...

On-Demand EC2 pricing
...
Region: Asia Pacific (Tokyo)

Compute instance type	Memory	vCPU	Linux price per build minute
arm1.small	3 GB	2	$0.00425
arm1.medium	8 GiB	4	$0.009