Use env flag for nosystemcrypto builders

[qmuntal]

Summary

• replace nosystemcrypto GOEXPERIMENT matrix entries with a dedicated nosystemcrypto builder flag
• include nosystemcrypto in generated builder IDs and display names
• set MS_GO_NOSYSTEMCRYPTO=1 in run-stage build/test steps for nosystemcrypto builders
• Remove experiment: systemcrypto flags. Systemcrypto is enabled by default, no need to explicitly set it.

Motivation

• GOEXPERIMENT=systemcrypto support will be removed in Go 1.27, see Remove GOEXPERIMENT=systemcrypto #2261.

Validation

• VS Code YAML diagnostics: no errors
• git diff --check

[Copilot]

Pull request overview

This PR switches nosystemcrypto CI coverage from GOEXPERIMENT=nosystemcrypto matrix entries to a dedicated builder flag that sets MS_GO_NOSYSTEMCRYPTO=1 during build/test execution.

Changes:

• Replaces nosystemcrypto matrix entries with nosystemcrypto: true and relies on default systemcrypto behavior otherwise.
• Propagates the new builder field into generated IDs and display names.
• Sets MS_GO_NOSYSTEMCRYPTO=1 for nosystemcrypto run-stage build/test steps.

Show a summary per file

File	Description
eng/pipeline/stages/go-builder-matrix-stages.yml	Updates builder matrix entries to use the new nosystemcrypto flag.
eng/pipeline/stages/shorthand-builders-to-builders.yml	Adds nosystemcrypto to builder schema, IDs, and display-name fragments.
eng/pipeline/stages/run-stage.yml	Displays nosystemcrypto builders and exports MS_GO_NOSYSTEMCRYPTO for relevant steps.
eng/pipeline/stages/run-codeql.yml	Updates schema/display-name handling for the new field.
eng/pipeline/stages/builders-to-stages.yml	Updates builder schema comments.
eng/pipeline/stages/sign-stage.yml	Updates builder schema comments.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 1