File Exporter: fixes for simple configuration

[winder]

Summary

Testing with a simple configuration wasn't working quite right:

Importer:
    Name: algod
    Config:
      - netaddr: {mainnet node addr}
        token: {token}
Exporter:
    Name: filewriter
    Config:
      - block-dir: {path}/data2/blocks

1. encoding failed when pointed to mainnet.
2. the metadata file was not being updated, leading to re-downloading the same blocks.

[codecov]

Codecov Report

Merging #1232 (214b5f7) into conduit (7cf43e8) will decrease coverage by 0.05%.
The diff coverage is 52.50%.

@@             Coverage Diff             @@
##           conduit    #1232      +/-   ##
===========================================
- Coverage    62.11%   62.06%   -0.06%     
===========================================
  Files           70       70              
  Lines         9408     9434      +26     
===========================================
+ Hits          5844     5855      +11     
- Misses        3063     3073      +10     
- Partials       501      506       +5     

Impacted Files	Coverage Δ
exporters/filewriter/file_exporter.go	64.40% <52.50%> (-6.25%)	⬇️

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