feat: add account number / sequence as env in txtar

[gfanton]

Pre-fetch the account number and sequence for both default and custom accounts created using the adduser and adduserfrom commands in txtar to make them available at runtime.
I've also converted the user environment variable to lowercase, as I feel this improves readability.

[Gno2D2]

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Codecov Report

Attention: Patch coverage is 79.48718% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gno.land/pkg/integration/testscript_gnoland.go	69.23%	16 Missing and 8 partials ⚠️

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[zivkovicmilos]

Great UX bump 👏

[zivkovicmilos]

Nice