Install ddev from master in the measure disk usage workflow

[lucia-sb]

What does this PR do?

Changes the "Measure Disk Usage" workflow to check out master first, install ddev from it, and only then check out the commit being measured.

Depends on #25325, which makes setup-ddev install ddev non-editably. Merge that one first: until it lands, the second checkout would replace the sources behind the editable install, and the measurement would run the measured commit's ddev.

Motivation

The workflow checks out the PR's/push's head_sha and then installs ddev "locally" from that same checkout. If a PR modifies ddev or datadog_checks_dev itself, the disk-usage measurement would run using the modified tool rather than a known-good version, which can produce unreliable or misleading measurements. Installing ddev from master instead decouples the measurement tool from the code being measured.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[cit-pr-commenter-54b7da]

evalya-impact-summary

evalya impact analysis

Impact analysis: RUN-ALL — every test task will run
Trigger:         empty diff (default branch, scheduled run, or shallow-clone fallback)
Test tasks:      0 (all selected)
Publish tasks:   2 (always emitted)
Diff:            empty (no diff information)

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[dd-octo-sts]

Disk usage change

Commit 61f695c compared against 1f98f39.

No integration or dependency changed size.

[dd-octo-sts]

Validation Report

All 21 validations passed.

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Validation	Description	Status
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config	Validate default configuration files against spec.yaml	✅
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imports	Validate check imports do not use deprecated modules	✅
integration-style	Validate check code style conventions	✅
jmx-metrics	Validate JMX metrics definition files and config	✅
labeler	Validate PR labeler config matches integration directories	✅
legacy-signature	Validate no integration uses the legacy Agent check signature	✅
license-headers	Validate Python files have proper license headers	✅
licenses	Validate third-party license attribution list	✅
metadata	Validate metadata.csv metric definitions	✅
models	Validate configuration data models match spec.yaml	✅
openmetrics	Validate OpenMetrics integrations disable the metric limit	✅
package	Validate Python package metadata and naming	✅
qa-label	Validate the pull request declares whether it needs QA for the next Agent release	✅
readmes	Validate README files have required sections	✅
saved-views	Validate saved view JSON file structure and fields	✅
version	Validate version consistency between package and changelog	✅

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