Export JITPACK_NDK_VERSION for jitpack

Jitpack build is failing with the following error

```
> Configure project :
Gradle version Gradle 7.1

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':app'.
> com.android.builder.errors.EvalIssueException: NDK from ndk.dir at /opt/android-sdk-linux/ndk-bundle had version [21.1.6352462] which disagrees with android.ndkVersion [22.1.7171670]
```

So attempting to manually export an env variable for jitpack which uses ndk 21.1.6352462 instead of the termux default 22.1.7171670 and which is also used by F-Droid

https://jitpack.io/com/github/termux/termux-app/0.115/build.log

https://github.com/jitpack/jitpack.io/blob/master/BUILDING.md#custom-commands

https://gitlab.com/fdroid/fdroiddata/-/blob/master/metadata/com.termux.yml#L726

Author: agnostic-apollo

app/build.gradle

@@ -4,7 +4,7 @@ plugins {
 
 android {
     compileSdkVersion project.properties.compileSdkVersion.toInteger()
-    ndkVersion project.properties.ndkVersion
+    ndkVersion = System.getenv("JITPACK_NDK_VERSION") ?: project.properties.ndkVersion
 
     dependencies {
         implementation "androidx.annotation:annotation:1.2.0"

jitpack.yml

@@ -0,0 +1,2 @@
+env:
+  JITPACK_NDK_VERSION: "21.1.6352462"

terminal-emulator/build.gradle

@@ -3,7 +3,7 @@ apply plugin: 'maven-publish'
 
 android {
     compileSdkVersion project.properties.compileSdkVersion.toInteger()
-    ndkVersion project.properties.ndkVersion
+    ndkVersion = System.getenv("JITPACK_NDK_VERSION") ?: project.properties.ndkVersion
 
     defaultConfig {
         minSdkVersion project.properties.minSdkVersion.toInteger()