Fix quotes around gcovr install command for Windows

[yennster]

Description

This PR changes the gcovr install command from pip install 'gcovr>=4.1' to pip install "gcovr>=4.1" to work successfully on Windows

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[cmonr]

Nice catch. Good 'ol Command Prompt bs Git Bash difference.

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3055
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8092/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2670
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8092/

[mbed-ci]

Test : FAILURE

Build number : 2832
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8092/2832

[cmonr]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 2840
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8092/2840