Add support for TEST_DNS_TXT_LEN secret in DNS API workflow (for acmetest PR)

[kubax]

Summary

This PR updates the DNS workflow to support the new TEST_DNS_TXT_LEN secret introduced in acmetest PR.
The related changes in the testing framework are implemented in my PR to acmetest:

acmetest PR: acmesh-official/acmetest#34

The updated acmetest adds the ability to configure the length of TXT values used during DNS API testing — a requirement for testing systems such as acme-dns, which expect TXT records with an exact length. To ensure acme.sh can pass this configuration to all test environments, the workflow needs to be updated accordingly.

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What This PR Does

1. Adds support for the TEST_DNS_TXT_LEN secret

• Introduces the new secret into all DNS test jobs
• Ensures that every environment (Linux/Docker, macOS, Windows/Cygwin, BSD variants, Solaris/OmniOS) passes the value through to letest.sh or the acmetest Docker runtime
• Keeps the value optional — if the secret is not defined, the default behavior remains unchanged

2. Updates Docker-based testing

• Adds TEST_DNS_TXT_LEN to docker.env so that the variable is available inside the acmetest container
• Ensures consistent behavior with the native runner environments

3. Updates VM-based testing (FreeBSD, OpenBSD, NetBSD, DragonFlyBSD, Solaris, OmniOS)

• Adds TEST_DNS_TXT_LEN to the envs: variable list so that the VM receives the configuration

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Why This Change Is Needed

The updated acmetest now allows generating TXT values of a configurable size.
This is required because some DNS ACME challenge implementations — specifically acme-dns — need TXT records of a fixed length to function correctly.

Without updating the acme.sh workflow:

• acmetest cannot receive the new configuration
• TXT length cannot be adjusted during CI
• DNS API tests relying on acme-dns will not work correctly
• the CI behavior between Docker and native platforms would become inconsistent

With this PR, the workflow fully supports the new functionality.

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Backward Compatibility

• If TEST_DNS_TXT_LEN is not defined, all workflows behave exactly as before.
• The default TXT length remains unchanged internally (handled by acmetest).
• No changes are required for existing DNS API providers or test setups.

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Summary

This PR ensures that acme.sh’s DNS testing workflow is compatible with the upcoming enhancements in acmetest by exposing the new TEST_DNS_TXT_LEN secret to all supported test environments.
It enables full and consistent testing of DNS providers that require fixed-length TXT values while preserving existing behavior for all others.