gitlab-ci: Only run nightly-i386-linux-deb9 when NIGHTLY is set

15 jobs for master in 255 minutes and 53 seconds (queued for 2 seconds)
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validate-x86_64-windows Full Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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validate-aarch64-linux-deb9 Full Build
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validate-x86_64-darwin Full Build
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validate-x86_64-windows-hadrian Full Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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circleci-validate-x86_64-darwin Full Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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