fix: handle missing git in GitHub Actions CI adapter

[sentry]

This PR addresses a FileNotFoundError that occurred when the codecov-cli was run in a GitHub Actions environment where git was not installed or not in the system's PATH.

The GithubActionsCIAdapter._get_commit_sha() method attempts to determine the parent commit SHA using git rev-parse HEAD^@ when a pull request number is detected. Previously, this subprocess.run call was not wrapped in any error handling. If git was not found, it would lead to an unhandled FileNotFoundError and crash the CLI.

The fix involves wrapping the subprocess.run call in a try/except OSError block. If an OSError (which includes FileNotFoundError) occurs, the CLI will now gracefully fall back to using the GITHUB_SHA environment variable, which is already the intended fallback for other scenarios where the git command might not yield the expected result. This prevents the CLI from crashing and ensures it can still report coverage in environments without git.

Fixes CLI-HB

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[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
723	1	722	1

View the full list of 1 ❄️ flaky test(s)

api.temp.calculator.test_calculator::test_divide

Flake rate in main: 100.00% (Passed 0 times, Failed 1726 times)

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

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