[servo] fix image build by setting HARFBUZZ_SYS_NO_PKG_CONFIG=1

[jankeromnes]

Fixes https://github.com/JanitorTechnology/janitor/issues/345

[jankeromnes]

@jdm could you please take a look at this fix?

Also, CI run should tell us if this actually fixes the Servo image build.

[jdm]

I'm not sure about the rules of when to use export or not, but that looks ok otherwise.

[jankeromnes]

Huh, I've never seen this build fail before:

Step 5/52 : RUN add-apt-repository ppa:git-core/ppa
 ---> Running in aeeb7f63f2b4
 The most current stable version of Git for Ubuntu.

For release candidates, go to https://launchpad.net/~git-core/+archive/candidate .
 More info: https://launchpad.net/~git-core/+archive/ubuntu/ppa
gpg: keyring `/tmp/tmpmkwcxqm1/secring.gpg' created
gpg: keyring `/tmp/tmpmkwcxqm1/pubring.gpg' created
gpg: requesting key E1DF1F24 from hkp server keyserver.ubuntu.com
gpgkeys: key E1DD270288B4E6030699E45FA1715D88E1DF1F24 can't be retrieved
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
gpg: keyserver communications error: keyserver helper general error
gpg: keyserver communications error: unknown pubkey algorithm
gpg: keyserver receive failed: unknown pubkey algorithm
Failed to add key.
The command '/bin/sh -c add-apt-repository ppa:git-core/ppa' returned a non-zero code: 1
Exited with code 1

(Totally unrelated to Servo, re-triggering again.)

[jankeromnes]

The fix worked!