fix: Fix/7.18.0 revert build script change (#8979)

## **Description**

<!--
Write a short description of the changes included in this pull request,
also include relevant motivation and context. Have in mind the following
questions:
1. What is the reason for the change?
2. What is the improvement/solution?
-->

Reverts PR #8919, which
broke E2E tests

## **Related issues**

Fixes:

## **Manual testing steps**

1. Go to this page...
2.
3.

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

<!-- [screenshots/recordings] -->

### **After**

<!-- [screenshots/recordings] -->

## **Pre-merge author checklist**

- [ ] I’ve followed [MetaMask Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've clearly explained what problem this PR is solving and how it
is solved.
- [ ] I've linked related issues
- [ ] I've included manual testing steps
- [ ] I've included screenshots/recordings if applicable
- [ ] I’ve included tests if applicable
- [ ] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.
- [ ] I’ve properly set the pull request status:
  - [ ] In case it's not yet "ready for review", I've set it to "draft".
- [ ] In case it's "ready for review", I've changed it from "draft" to
"non-draft".

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

README.md

@@ -31,6 +31,7 @@ Before running the app, make sure your development environment has all the requi
 ### Building the app
 
 **Clone the project**
+
 ```bash
 git clone git@github.com:MetaMask/metamask-mobile.git && \
 cd metamask-mobile
@@ -44,22 +45,32 @@ yarn setup
 
 _Not the usual install command, this will run scripts and a lengthy postinstall flow_
 
-_Not the usual install command, this will run scripts and a lengthy postinstall flow_
+**Setup environment variables**
+
+```bash
+cp .ios.env.example .ios.env && \
+cp .android.env.example .android.env && \
+cp .js.env.example .js.env
+```
 
 ### Running the app
 
 **Run Metro bundler**
+
 ```bash
 yarn watch
 ```
+
 _Like a local server for the app_
 
 **Run on a iOS device**
+
 ```bash
-yarn start:ios 
+yarn start:ios
 ```
 
 **Run on an Android device**
+
 ```bash
 yarn start:android
 ```

scripts/postinstall.sh

@@ -22,17 +22,3 @@ echo "" > ios/release.xcconfig
 echo "6. Init git submodules"
 echo "This may take a while..."
 git submodule update --init
-
-echo "7. Copy env variables..."
-  if [ ! -f .js.env ]; then
-      echo "Creating .js.env..."
-      cp .js.env.example .js.env
-  fi
-  if [ ! -f .android.env ]; then
-      echo "Creating .android.env..."
-      cp .android.env.example .android.env
-  fi
-  if [ ! -f .ios.env ]; then
-      echo "Creating .ios.env"
-      cp .ios.env.example .ios.env
-  fi