CI image golang update to 1.15.7

[whoisxx]

Proposed changes

• klaytn CI, build image golang update to 1.15.7

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• Bugfix
• New feature or enhancement
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Further comments

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[aidan-kwon]

@whoisxx The test failure will be resolved when #876 is merged.

[KimKyungup]

@whoisxx Rebase please. :)

[whoisxx]

@aidan-kwon thank you. it work~!