Set the target when packaging for magicleap

[asajeffrey]

If we don't set the target during packaging, we get errors checking for gstreamer.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes do not require tests because it fixes packaging errors.

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @wafflespeanut: python/servo/package_commands.py

[asajeffrey]

As seen at #23114:

$ MAGICLEAP_SDK=/Users/ajeffrey/MagicLeap/mlsdk/v0.19.0 MLCERT=/Users/ajeffrey/MagicLeap/cert/alanjeffreymlcert.cert ./mach package -r --magicleap
Error running mach:

    ['package', '-r', '--magicleap']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.

You should consider filing a bug for this issue.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

Exception: Your system's gstreamer libraries are out of date (we need at least 1.12). If you're unable to install them, let us know by filing a bug!

  File "/Users/ajeffrey/github/asajeffrey/servo/python/servo/package_commands.py", line 215, in package
    env = self.build_env(target=target)
  File "/Users/ajeffrey/github/asajeffrey/servo/python/servo/command_base.py", line 612, in build_env
    if self.needs_gstreamer_env(target):
  File "/Users/ajeffrey/github/asajeffrey/servo/python/servo/command_base.py", line 560, in needs_gstreamer_env
    install them, let us know by filing a bug!")

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit 733ff95 has been approved by jdm

[bors-servo]

⌛ Testing commit 733ff95 with merge cd8b9ca...

[bors-servo]

☀️ Test successful - android-mac, arm32, arm64, linux-rel-css, linux-rel-wpt, mac-rel-css1, mac-rel-css2, mac-rel-wpt1, mac-rel-wpt2, mac-rel-wpt3, mac-rel-wpt4, magicleap, status-taskcluster
Approved by: jdm
Pushing cd8b9ca to master...