Overwrite the last servo-latest.ext with current file

[edunham]

So we kinda changed the naming scheme at the last minute and this PR will make it so we actually upload packages to the location (servo-latest.ext) that we're telling people to download them from.

If none of this makes any sense, please ping me in the morning and I'll fix it all up and be mildly embarrassed :)

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• ./mach build -d does not report any errors
  /etc/ci is unrelated to the build
• ./mach test-tidy does not report any errors
  It's all mad about some wpt stuff I never touched
• These changes fix #__ (github issue number if applicable).
  IRC conversation

• There are tests for these changes OR
• These changes do not require tests because I hope someone fluent in bash takes a look and tells me if they're bad

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This change is

[highfive]

Heads up! This PR modifies the following files:

• @aneeshusa: etc/ci/upload_nightly.sh

[aneeshusa]

nit: don't have the ${ext} variables include the period but write "servo-latest.${3}"

[aneeshusa]

Put the ext= lines above the package= lines, and use the ext variable to keep the extension in sync (i.e. when we have .msi for windows).

[larsbergstrom]

@edunham do you mind if we close this in favor of #12004 from a contributor?

[edunham]

@larsbergstrom thanks for catching the dup; #12004 looks like a more robust and higher quality script than I threw together last night :)