fix(gatsby-plugin-google-tagmanager): fix custom dataLayer name

[chilic]

Description

This PR is fix regression for #12783
After #11379 was merged.

Affected code: https://github.com/gatsbyjs/gatsby/pull/11379/files#diff-858da3c14fa621fa9060ba6403b9d317L30

Related Issues

Related to #12782

[wardpeet]

Sweet! 🎉 Thank you for fixing this. Code change looks :ok-hand:!

💜

[gatsbot]

Holy buckets, @chilic — we just merged your PR to Gatsby! 💪💜

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