[utils] update-checkout: Reduce environment variable noise

[lorentey]

This stops unnecessary overriding and printing every environment variable when cloning repositories.

(This is a followup to #41714)

[lorentey]

Before:

+ env GIT_TERMINAL_PROMPT=0 HOME=/Users/klorentey LANG=en_US.UTF-8 LESS=-R \
LOGNAME=klorentey LaunchInstanceID=*** PATH=*** PWD=/Users/klorentey/Swift/swift \
SECURITYSESSIONID=*** SHELL=/opt/brew/bin/fish SHLVL=1 \ SSH_AUTH_SOCK=/private/tmp/com.apple.launchd.***/Listeners TERM=xterm-256color \
TERM_PROGRAM=Apple_Terminal TERM_PROGRAM_VERSION=444 \
TERM_SESSION_ID=AE93EAEA-A0A0-42F1-A767-************ TMPDIR=/var/folders/tg/***/T/ \
USER=klorentey XPC_FLAGS=0x0 XPC_SERVICE_NAME=0 __CFBundleIdentifier=com.apple.Terminal \
__CF_USER_TEXT_ENCODING=0x1F5:0x0:0x0 \
git clone --recursive [email protected]:apple/swift-collections.git swift-collections

After:

+ env GIT_TERMINAL_PROMPT=0 git clone \
--recursive [email protected]:apple/swift-collections.git swift-collections

[lorentey]

@swift-ci smoke test

[lorentey]

I love linters.

******************** TEST 'Swift-validation(macosx-x86_64) :: Python/python_lint.swift' FAILED ********************
Script:
--
: 'RUN: at line 8';   /Applications/Xcode-beta.app/Contents/Developer/usr/bin/python3 /Users/ec2-user/jenkins/workspace/swift-PR-macos-smoke-test/branch-main/swift/utils/python_lint.py
--
Exit Code: 1

Command Output (stdout):
--
./utils/update_checkout/update_checkout/update_checkout.py:287:12: E201 whitespace after '{'
./utils/update_checkout/update_checkout/update_checkout.py:287:37: E202 whitespace before '}'

--

********************

[lorentey]

@swift-ci smoke test and merge

[lorentey]

<EXPR>:0: error: TestToolTests.testEnableDisableTestability : threw error "signalled(11): /home/build-user/build/buildbot_linux/swiftpm-linux-x86_64/x86_64-unknown-linux-gnu/release/swift-test --package-path /tmp/Miscellaneous_TestableExe.porkiV/Miscellaneous_TestableExe --enable-testable-imports --vv output:

[lorentey]

@swift-ci smoke test Linux platform

[lorentey]

@swift-ci smoke test

[lorentey]

Hm, 🤔

+ env GIT_TERMINAL_PROMPT=0 git clone --recursive https://github.com/KitWare/CMake.git cmake
Cloning into 'cmake'...
fatal: unable to access 'https://github.com/KitWare/CMake.git/': Failed to connect to github.com port 443 after 131117 ms: Connection timed out

[lorentey]

@swift-ci smoke test linux platform