fix(sam): set working directory for sam build #2755

Problem:
All of our SAM CLI invocations are ran without an explicit working directory.
This creates tons of hard-to-diagnose problems.

Solution:
* Explicitly set a working directory for `sam build` to the template's
  directory. This mimics the typical situation when running `sam build` in
  a terminal.
    * This correctly creates a `.aws-sam` directory adjacent to the target
      template.
* Other `sam` commands are still affected although they are harder to change
  without breaking things due to the temporary directory :/ 
* The way the Toolkit + SAM CLI interact is incredibly brittle and this
  solution is just a mitigation.

ref #2050
ref #2395

Author: JadenSimon

src/shared/sam/cli/samCliBuild.ts

@@ -10,6 +10,7 @@ import { pushIf } from '../../utilities/collectionUtils'
 import { localize } from '../../utilities/vsCodeUtils'
 import { Timeout, waitTimeout } from '../../utilities/timeoutUtils'
 import { ChildProcessResult } from '../../utilities/childProcess'
+import { dirname } from 'path'
 
 export interface SamCliBuildInvocationArguments {
     /**
@@ -133,7 +134,10 @@ export class SamCliBuildInvocation {
         }
 
         let childProcessResult: Promise<ChildProcessResult | void> = this.args.invoker.invoke({
-            spawnOptions: { env },
+            spawnOptions: {
+                env,
+                cwd: dirname(this.args.templatePath),
+            },
             arguments: invokeArgs,
             onStdout: onOutput,
             onStderr: onOutput,