ci: Use pip install editable mode

Due to the usage of
`
use_scm_version = {
    "root": "../..",
    "relative_to": __file__
},
in `setup.py`, `pip install .` fails in CI. Switch to using editable mode of
`pip install` to avoid the this error.

Author: urutva

azure-pipelines.yml

@@ -154,15 +154,15 @@ stages:
           - script: "pip install ."
             displayName: "Ensure mbed-os-tools can be installed locally for development"
 
-          - script: "pip install ."
+          - script: "pip install -e ."
             displayName: "Ensure mbed-ls can be installed locally for development"
             workingDirectory: "packages/mbed-ls"
 
-          - script: "pip install ."
+          - script: "pip install -e ."
             displayName: "Ensure mbed-host-tests can be installed locally for development"
             workingDirectory: "packages/mbed-host-tests"
 
-          - script: "pip install ."
+          - script: "pip install -e ."
             displayName: "Ensure mbed-greentea can be installed locally for development"
             workingDirectory: "packages/mbed-greentea"