Reproducibility of the export pipeline

## Context of the issue

I used tpot to fit my dataset, I got the different export pipeline for each run.

## Process to reproduce the issue

The steps for generating exported pipeline, the shape of my dataset was (45, 478).
```
X_train, X_test, y_train, y_test = \
sklearn.model_selection.train_test_split(X, y, random_state=1,test_size = 0.15)
M1 = TPOTRegressor(generations=10, population_size=40, verbosity=2, random_state=42,n_jobs =-1,cv=5)
M1.fit(X_train, y_train)
M1.export('M1_pipeline.py')
```

## Current result
1. When I firstly ran, the export pipeline was DecisionTreeRegressor
```
Generation 1 - Current best internal CV score: -0.6631261058133652
Generation 2 - Current best internal CV score: -0.6631261058133652
Generation 3 - Current best internal CV score: -0.6442071896861652
Generation 4 - Current best internal CV score: -0.5726875496699182
Generation 5 - Current best internal CV score: -0.5726875496699182
Generation 6 - Current best internal CV score: -0.528473933017039
Generation 7 - Current best internal CV score: -0.528473933017039
Generation 8 - Current best internal CV score: -0.528473933017039
Generation 9 - Current best internal CV score: -0.528473933017039
Generation 10 - Current best internal CV score: -0.528473933017039

Best pipeline: DecisionTreeRegressor(Normalizer(input_matrix, norm=max), max_depth=3, min_samples_leaf=10, min_samples_split=9)
```
2. When I secondly ran, the export pipeline was ExtraTreesRegressor
```
Generation 1 - Current best internal CV score: -0.6631261058133652
Generation 2 - Current best internal CV score: -0.6631261058133652
Generation 3 - Current best internal CV score: -0.6593793694494272
Generation 4 - Current best internal CV score: -0.6524528603774085
Generation 5 - Current best internal CV score: -0.636417747633282
Generation 6 - Current best internal CV score: -0.633586381252542
Generation 7 - Current best internal CV score: -0.633586381252542
Generation 8 - Current best internal CV score: -0.633586381252542
Generation 9 - Current best internal CV score: -0.633586381252542
Generation 10 - Current best internal CV score: -0.633586381252542

Best pipeline: ExtraTreesRegressor(LinearSVR(input_matrix, C=1.0, dual=True, epsilon=0.01, loss=epsilon_insensitive, tol=1e-05), bootstrap=False, max_features=0.3, min_samples_leaf=6, min_samples_split=13, n_estimators=100)
```

## Expected result
I would like to have a repeatable and stable export pipeline. The environment version I am using is Python 3.7.12, TPOT 0.11.7.

Thank you very much for the development and maintenance of TPOT.