plugins: manual_download: Resolve copyFile promise

[ix5]

The whole block beginning with .then(downloadedFilePath and ending with copyFile() (^1) was never going to resolve its promise with an actual return value.
That meant that that checkFile() was being called before any of the promise functions inside the previous block had
the chance to run, therefore the copied file was not yet available, leading to checkFile trying to verify a non-existent path, leading to a checksum mismatch error.

Solution is to make the whole block return something, in this case the value of the last chained promise.

Solves:

error: Error: core:manual_download: Error: checksum mismatch

Fixes #1754

^1:

ubports-installer/src/core/plugins/core/plugin.js

Lines 205 to 219 in 95745e5

	.then(downloadedFilePath => {
	fs.ensureDir(
	path.join(this.cachePath, this.props.config.codename, group)
	).then(() =>
	fs.copyFile(
	downloadedFilePath,
	path.join(
	this.cachePath,
	this.props.config.codename,
	group,
	file.name
	)
	)
	);
	})

[codecov-io]

Codecov Report

Merging #1786 (974468a) into master (95745e5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1786   +/-   ##
=======================================
  Coverage   57.62%   57.62%           
=======================================
  Files          26       26           
  Lines         800      800           
=======================================
  Hits          461      461           
  Misses        339      339           

Impacted Files	Coverage Δ
src/core/plugins/core/plugin.js	100.00% <100.00%> (ø)

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[ix5]

Pushed the lint fix in a separate commit

[ix5]

@NeoTheThird @UniversalSuperBox mind taking a look at this?

[ix5]

Since Jan seems MIA currently, @Flohack74 would you like to review this?

[NeoTheThird]

lgtm, sorry for the delay and thanks for the contribution

[ix5]

lgtm, sorry for the delay and thanks for the contribution

Hey, you're under no obligation here. Thanks for writing this installer in the first place!