Fix issues loading data-dir value from env vars or dropin config files

[brandond]

Proposed Changes

• Don't set K3S_DATA_DIR env var in self-extracting multicall wrapper
  This was only used to pass the bundled strongswan path through to the flannel ipsec backend, and is no longer needed. Ref: Initial encrypted network support #719
• Add K3S_DATA_DIR as env var for --data-dir flag
• Fix inconsistent loading of config dropins when config file does not exist
  configfilearg.FindString would silently skip parsing dropins if the main config file didn't exist. If a custom config file path was passed it would raise an error, but if we were parsing the default config file and it didn't exist it would just silently fail to load the dropins.

Types of Changes

bugfix

Verification

Includes tests

Testing

See linked issue

Linked Issues

• K3s does not respect data-dir set via config file drop-in, or via env var #10589

User-Facing Change

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 43.47%. Comparing base (59e0761) to head (f07f281).

Files	Patch %	Lines
pkg/configfilearg/parser.go	91.17%	1 Missing and 2 partials ⚠️

❗ There is a different number of reports uploaded between BASE (59e0761) and HEAD (f07f281). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (59e0761)	HEAD (f07f281)
e2etests	8	6

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10591      +/-   ##
==========================================
- Coverage   49.49%   43.47%   -6.02%     
==========================================
  Files         179      179              
  Lines       14957    14950       -7     
==========================================
- Hits         7403     6500     -903     
- Misses       6194     7255    +1061     
+ Partials     1360     1195     -165     

Flag	Coverage Δ
e2etests	36.22% <13.88%> (-10.16%)	⬇️
inttests	36.63% <30.55%> (+16.94%)	⬆️
unittests	13.40% <86.11%> (+0.01%)	⬆️

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