Perf Test: Add missing copy step

[JasonGore]

Pull request checklist

• Addresses an existing issue: Fixes #0000
• Include a change request file using $ npm run change

Description of changes

The results of #9516 exposed an issue where a missing copy step caused the PR to run against an old version of perf-test. We initially removed this copy step in #9550 because we thought it would cause an error on the second copy including test results, but didn't consider this would result in PRs using an old copy of index.html.

Microsoft Reviewers: Open in CodeFlow

[size-auditor]

Size Auditor did not detect a change in bundle size for any component!

[msft-github-bot]

Component Perf Analysis:

Scenario	Master Samples *	PR Samples *
BaseButton	880	890
BaseButtonNew	3695	3663
DefaultButton	1169	1196
DefaultButtonNew	3239	3256
DetailsRow	8563	8524
DetailsRowNoStyles	6353	6424
DocumentCardTitle	43288	42327
MenuButton	1920	1963
MenuButtonNew	6092	6123
PrimaryButton	1320	1346
PrimaryButtonNew	3513	3705
SplitButton	3920	3805
SplitButtonNew	13791	13290
Toggle	2007	1969
ToggleNew	2457	2437
button	76	85

* Sample counts can vary by up to 30% and shouldn't be used solely for determining regression. Flamegraph links are provided to give a hint on deltas introduced by PRs and potential bottlenecks.

[msft-github-bot]

Hello @cliffkoh!

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[cliffkoh]

@msft-github-bot merge in 1 hour

[msft-github-bot]

Hello @cliffkoh!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I won't merge this pull request until after the UTC date Tue, 25 Jun 2019 19:16:28 GMT, which is in 1 hour

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