correct flags for log file

[seiyab]

Trying grype, I found that it overwrites previous content in log file without truncating. So in some situations, log file can be broken. Feel free to ask me to use other flags like O_APPEND or O_EXCL.

Reproduction:

printf 'previous content\n%.0s' {1..10} > my-log.txt
GRYPE_LOG_LEVEL=info GRYPE_LOG_FILE=./my-log.txt go run ./cmd/grype alpine

Resulting my-log.txt:

[0000]  INFO grype version: [not provided]
[0001]  INFO found 12 vulnerability matches across 15 packages
ious content
previous content
previous content
previous content

Merging this, resulting my-log.txt wil be:

[0000]  INFO grype version: [not provided]
[0001]  INFO found 12 vulnerability matches across 15 packages

[kzantow]

This makes sense to me @seiyab; thanks! Since we're already overwriting any existing file here, truncating it seems like the right thing to do if it exists.