Update the link to the latest OS image

[Suikaba]

What this patch does to fix the issue.

Update the link to the latest OS image (ubuntu 18.04).

Link to any relevant issues or pull requests.

Closes #1169

[bo-code-review-bot]

This PR needs Approvals as follows.

• Ownership Approval for / from iizukak, tkng, ruimashita

Please choose reviewers and requet reviews!

Click to see how to approve each reviews

You can approve this PR by triggered comments as follows.

• Approve all reviews requested to you (readability and ownership) and LGTM review
  Approval, LGTM

• Approve all ownership reviews
  Ownership Approval or OA

• Approve all readability reviews
  Readability Approval or RA

• Approve specified review targets

  • Example of Ownership Reviewer of /: Ownership Approval for / or OA for /
  • Example of Readability Reviewer of Python: Readability Approval for Python or RA for Python

• Approve LGTM review
  LGTM

See all trigger comments

Please replace [Target] to review target

• Ownership Approval
  • Ownership Approval for [Target]
  • OA for [Target]
  • Ownership Approval
  • OA
  • Approval
• Readability Approval
  • Readability Approval for [Target]
  • RA for [Target]
  • [Target] Readability Approval
  • [Target] RA
  • Readability Approval
  • RA
  • Approval
• LGTM
  • LGTM
  • lgtm

[iizukak]

OA

[iizukak]

/ready

[bo-mergebot]

⏳Merge job is queued...