Explicitly disable externally_connectable in manifest

[lukaw3d]

From oasis-wallet-ext oasisprotocol/oasis-wallet-ext#282 (comment)

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ EDITORCONFIG	editorconfig-checker	1		0	0.0s
✅ JSON	eslint-plugin-jsonc	1	0	0	0.97s
✅ JSON	jsonlint	1		0	0.21s
✅ JSON	prettier	1	0	0	0.45s
✅ JSON	v8r	1		0	1.75s
✅ REPOSITORY	checkov	yes		no	11.91s
✅ REPOSITORY	git_diff	yes		no	0.0s

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[codecov]

Codecov Report

Merging #1109 (9b0b403) into master (2ef70ef) will decrease coverage by 5.08%.
The diff coverage is 64.92%.

@@            Coverage Diff             @@
##           master    #1109      +/-   ##
==========================================
- Coverage   88.77%   83.68%   -5.09%     
==========================================
  Files         103      121      +18     
  Lines        1781     2249     +468     
  Branches      411      539     +128     
==========================================
+ Hits         1581     1882     +301     
- Misses        200      367     +167     

Flag	Coverage Δ
cypress	51.80% <23.04%> (-8.84%)	⬇️
jest	76.88% <64.72%> (-1.91%)	⬇️

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Impacted Files	Coverage Δ
src/app/components/ErrorFormatter/index.tsx	100.00% <ø> (ø)
src/app/pages/AccountPage/index.tsx	95.16% <ø> (ø)
src/app/state/transaction/index.ts	82.85% <0.00%> (-2.44%)	⬇️
src/store/reducers.ts	100.00% <ø> (ø)
src/store/sagas.ts	100.00% <ø> (ø)
src/styles/theme/ThemeProvider.tsx	100.00% <ø> (ø)
src/types/errors.ts	50.00% <ø> (ø)
src/app/pages/ParaTimesPage/index.tsx	6.45% <6.45%> (ø)
src/app/state/paratimes/index.ts	7.14% <7.14%> (ø)
src/app/lib/eth-helpers.ts	30.76% <30.76%> (ø)
... and 27 more