fix(rust): use "dashed" form for systemd-firstboot keymap

[imobachgs]

Problem

When writing the keymap to be used by the target system, Agama does not uses the "dashed" form:
"es(ast)" instead of es-ast.

• bsc#1236174

Solution

Use the "dashed" form in that case so systemd-firstboot is able to handle the keyboard.

[dgdavid]

I miss a unit test, but not sure how you're doing that in Rust.

Code LGTM anyway.

[coveralls]

Pull Request Test Coverage Report for Build 13005419087

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-18.2%) to 70.932%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rust/agama-server/src/l10n/model.rs	0	1	0.0%

Totals
Change from base Build 12995156707:	-18.2%
Covered Lines:	17299
Relevant Lines:	24388

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💛 - Coveralls

[imobachgs]

In this case what I really miss is an integration test to be sure that everything is set up as expected after the first boot.