Fix ReferenceError (#2434)

When OTEL_TRACES_SAMPLER is used without OTEL_TRACES_SAMPLER_ARG there would be a ReferenceError due to getSamplerProbabilityFromEnv being called above the definition of DEFAULT_RATIO.

```
ReferenceError: Cannot access 'DEFAULT_RATIO' before initialization
    at null.getSamplerProbabilityFromEnv (/app/node_modules/@opentelemetry/tracing/src/config.ts:91:58)
    at null.buildSamplerFromEnv (/app/node_modules/@opentelemetry/tracing/src/config.ts:71:44)
    at Object.<anonymous> (/app/node_modules/@opentelemetry/tracing/src/config.ts:38:12)
    at Module._compile (internal/modules/cjs/loader.js:1085:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)
    at Module.load (internal/modules/cjs/loader.js:950:32)
    at Function.Module._load (internal/modules/cjs/loader.js:784:16)
    at Module.require (internal/modules/cjs/loader.js:974:19)
    at require (internal/modules/cjs/helpers.js:92:18)
    at Object.<anonymous> (/app/node_modules/@opentelemetry/tracing/src/utility.ts:17:1)```

packages/opentelemetry-sdk-trace-base/src/config.ts

@@ -27,6 +27,7 @@ import {
 
 const env = getEnv();
 const FALLBACK_OTEL_TRACES_SAMPLER = TracesSamplerValues.AlwaysOn;
+const DEFAULT_RATIO = 1;
 
 /**
  * Default configuration. For fields with primitive values, any user-provided
@@ -79,8 +80,6 @@ export function buildSamplerFromEnv(
   }
 }
 
-const DEFAULT_RATIO = 1;
-
 function getSamplerProbabilityFromEnv(
   env: Required<ENVIRONMENT>
 ): number | undefined {